Plant Breeding as a Hobby
Breeding plants to create new varieties and improve upon old ones is a
hobby that nearly everyone can engage in. The crossing techniques are easy
to learn and you can experiment with many kinds of plants. Generally, amateur
plant breeders work with traits that are fairly easy to change -- for example,
flower color, fruit shape, or plant size. Nevertheless, although your experiment
may be simple, it is possible for you to produce unusual or beautiful plants.
In order to breed plants successfully it is important to understand
the principles of plant reproduction. The purpose of this circular is to
explain these principles and to describe some of the simple techniques
that you can use to produce new varieties or strains of plants.
Plant characteristics can be changed after many generations by a process
of selection. There are two types of selection -- natural and artificial.
Natural selection is the process that occurs in nature whereby
strong and well- adapted plants survive while weak and poorly adapted plants
eventually die out. This process has taken place since the beginning of
life on earth and it is still occurring in nature.
Artificial selection is the process that humans use to obtain
more desirable types of plants. Thousands of years ago people learned that
saving seed from the kind of plant they wanted to continue growing would
increase the chances of getting a plant similar to the original. But our
ancestors didn't know what their chances of success were nor did they understand
the processes by which traits were changed or maintained. It wasn't until
the eighteenth and nineteenth centuries that humans began to understand
the laws of heredity and the processes of plant reproduction. Even today
these fundamentals aren't completely understood. But enough is known so
that we can select plants for breeding with considerably more assurance
of success than our primitive ancestors did.
Fundamentals of Plant Reproduction
Kinds of reproduction
Plants reproduce in two ways -- asexually and sexually.
Asexual, or vegetative, reproduction occurs without the fusion
of germ (reproductive) cells. In garden plants, asexual reproduction occurs
when a part of the plant is separated from the parent plant and develops
into a complete plant, as when strawberries produce runners which take
root and form new strawberry plants. Asexual reproduction can be brought
about artificially by means of leaf cuttings, stem cuttings, root cuttings,
etc. Plants that originate from asexual reproduction are usually identical
to the parent plant.
Sexual reproduction involves the union of a male and a female
germ cell. From this union a seed -- and ultimately a new plant -- is produced.
Sexual reproduction is the most common type of reproduction for garden
plants. The plants originating from sexual reproduction are often quite
different from their parents and from each other. Because of this possibility
for variation, sexual reproduction of plants is the method used by plant
breeders in developing new strains and varieties.
Parts of the flower
Sexual reproduction of plants is carried on in the flower. The reproductive
organs of the flower are the stamen and pistil (Fig. 1). A part of the
stamen called the anther produces pollen grains, which contain male germ
cells. At the base of the pistil is the ovary; it produces ovules, which
contain the female germ cells. When the ovules are fertilized by the male
germ cells, they develop into seeds in the ovary.
||Parts of a flower. The flower shown here is a perfect flower; that
is, it has male and female reproductive organs. The stamen is the male
organ and the pistal is the female organ. нн
The petals, which add greatly to the beauty of flowers, serve such useful
functions as aiding in pollination and protecting the sex organs from physical
damage. The sepals, which often are green, cuplike structures, support
the petals and protect the floral parts.
The flower parts vary greatly in size, shape, color, and number, but
generally it is easy to distinguish the pistil from the stamens. You can
identify the stamens by the yellow powder (pollen) on their tips and the
pistil by a bulge (the ovary) at its base. On an unusual flower, however,
it might require careful study to tell them apart.
Types of flowers
Usually a plant's reproductive organs are arranged in one of three ways
-- as a perfect flower, as an imperfect flower, or as a composite flower.
A perfect flower contains both stamens and a pistil (Fig.
1) It is the most common type of flower. Some common perfect flowers
are those of tomatoes, morning-glories, snapdragons, petunias, lilies,
An imperfect flower (Fig. 2) contains only one type of sex organ.
Those containing the pistil are called pistillate flowers; those containing
stamens are called staminate flowers. Both pistillate and staminate flowers
may grow on the same plant, or the staminate flower may be on one plant
and the pistillate flower on another. Examples of plants with pistillate
and staminate flowers on the same plant are squash, corn, clematis, bittersweet,
and begonias. Plants with staminate and pistillate flowers on different
plants include asparagus, spinach, willow, and cottonwood.
||The flowers of squash are imperfect, each flower having only one
type of sex organ. нн
A composite flower (Fig. 3) is actually a cluster of small flowers,
called florets, joined together in what is called a flower head. The florets
often resemble petals. Some florets have both stamens and pistils; they
are called disc florets. Some have only pistils; they are called ray florets.
A composite flower can be made up of only disc florets or of a combination
of disc and ray florets. Typical composite flowers are zinnias, chrysanthemums,
asters, and marigolds.
|Cross section of a chrysanthemum. Composite flowers, such as chrysanthemums,
are made up of clusters of florets. Some composite flowers have both ray
and disc florets while some have only ray florets. нн
Pollination and Fertilization
Pollination is the transfer of pollen from an anther to a stigma.
When the anther is mature it splits open and discharges the pollen. The
pollen is then carried to the stigma by various natural means, the most
common being wind and insects. In plant breeding, pollination is carefully
controlled by humans.
There are two kinds of pollen transfer -- cross-pollination and self-
pollination. In cross-pollination the pollen is transferred from the anther
of a flower of one plant to the stigma of a flower of another plant. In
self-pollination the pollen is transferred from the anther to the stigma
of the same flower or to another flower of the same plant.
Fertilization is the uniting of two germ cells. After the pollen
has come to rest on the stigma, under favorable conditions the pollen develops
a tube which grows downward through the style and into the ovule (Fig.
4). It is in the ovule that the male and female germ cells unite. Once
fertilization has occurred development of the ovule begins. The result
is a seed.
||Fertilization takes place when the male and female cells unite in
the ovule. нн
The parent flower that furnishes the pollen is called the pollen parent;
the one that bears the seed is called the seed parent.
Plants inherit characteristics from their parents in the same way that
animals do. The laws of heredity explain why different traits are inherited
by offspring of the same parents. Moreover, these laws make it possible
to predict the number of offspring that will inherit a certain trait. A
knowledge of the laws of heredity is essential for effective plant breeding.
Genes. All cells contain genes, which are the units that determine
the traits of a plant or animal. Except for the germ cells, each cell in
a plant contains two genes for each trait -- for example, for the trait
of color a plant could have one gene for yellow and one for red. The germ
cells have only one gene for each trait. Of the thousands of germ cells
that a plant produces, about half have one of the genes for each trait
and about half have the other gene.
When the male and female germ cells unite in the ovule each contributes
one gene for each trait so that the new seed then has two genes for each
trait. The various combinations of the many genes inherited from the pollen
parent and from the seed parent determine the traits of the offspring and
of future generations.
You can't tell by looking at a plant exactly what genes it contains.
For example, a plant may have one gene for red and one for yellow, but
only a red color may show in its flower; by looking at the plant you have
no way of knowing about the gene for yellow. Nevertheless, the gene for
yellow is present in the plant's cells and some of the plant's offspring
will inherit this gene, which they in turn will pass on to their offspring.
If the plant and its offspring are self-pollinated, in the first or second
generations following, depending on how the genes interact, some of the
offspring will produce yellow flowers. By observing the occurrence in the
offspring of different colors (or other traits that you are interested
in) you can find out what genes the parent plants have and how they interact.
With this knowledge, your plant-breeding job will be greatly simplified.
If a plant that is self-pollinated produces offspring identical to itself,
then it is said to breed true. If the first-generation offspring are not
all identical to the parent plant, then it is said that segregation has
How genes interact. In the following discussion, we will assume
that we have crossed two true-breeding plants, one having two genes for
red-colored flowers and the other having two genes for yellow-colored flowers.
The offspring of this cross are called the first filial or F1
generation. Figure 5 illustrates the ways in which the genes of the pollen
parent can combine with those of the seed parent. Each square represents
one seed. In the diagram the letter R represents the gene for red
color and the letter rrepresents the gene for yellow color. As shown
in Fig. 5, each F1 plant will inherit a gene for red from one
parent and a gene for yellow from the other parent.
||Each square represents one seed of the F1 generation.
Each seed inherits a gene for red (R) from one parent and a gene for yellow
(r) from the other. нн
When the F1 plants are self-pollinated, or "selfed," their
genes can combine in four different ways. Figure 6 illustrates the four
possible combinations. The offspring resulting from the self-pollination
of the F1 generation are called the second filial or F2
As shown in Fig. 6, out of every four seeds that are produced we may expect
one seed that will have two genes for red (RR), two seeds that will have
one gene for red and one gene for yellow (Rr), and one seed that will have
two genes for yellow (rr). (This ratio, like all ratios concerning probability,
is based on large numbers. If there are only a small number of seeds, the
ratio may vary somewhat from the one given above. However, for large numbers
of seeds, the above ratio will hold true.)
||When the F1 plant is selfed segregation occurs, and the
F2 generation inherits traits in a 1:2:1 ratio (1 RR, 2Rr, 1rr).
If a seed contains two genes for the same color, it is said to be pure
for that trait and the resulting flower will show that color. For example,
in Fig. 6 the seeds containing two genes for the color red (RR) will result
in red flowers and the seeds containing two genes for yellow (rr) will
result in yellow flowers. The seeds containing genes for both colors (Rr)
may result either in flowers that are red or in flowers that are mauve
(a shade halfway between red and yellow), depending on whether the gene
for red has complete or incomplete dominance.
Complete dominance. Some genes are dominant and some are recessive.
Whenever a dominant and a recessive gene for a particular trait occur in
the same plant, the dominant gene will cause its character to show up in
the plant to the exclusion of the recessive trait.
In Figs. 5 and 6, let us assume
that red (R) is dominant and yellow (r) is recessive. Whenever an R occurs,
even if an r also occurs, we can expect the flower to be red. In Fig. 5
(F1generation) all the seeds have an R gene; thus all the flowers
will be red. In Fig. 6 (F2generation) R occurs in three of the
squares; these three seeds will result in red flowers. The dominant gene
R does not appear in the lower right square; therefore the recessive trait
yellow will show in the flowers of this plant. Thus, for a large number
of plants in the F2 generation, where red has complete dominance
over yellow the color ratio will be 3:1, that is, three red to one yellow
Incomplete dominance. Sometimes neither gene is dominant or recessive
and when they occur in the same plant, they form an intermediate characteristic.
If we assume incomplete dominance in Figs. 5 and 6, then wherever Rr occurs
we can expect the flower to be mauve. An examination of Figs. 5
and 6 shows that in theF1 generation all
the flowers will be mauve while in the F2 generation we may
expect one red (RR), two mauve (Rr), and one yellow (rr) flower out of
every four -- a 1:2:1 ratio (Fig. 8).
|If red has complete dominance over yellow, then the seeds represented
in Fig. 6 will develop flowers having a color ratio of 3:1 - out of every
four floweres, three will be red and one will be yellow. нн
Fixing a trait. Cross-pollination is usually necessary to develop
new varieties. Once the trait that is desired is fixed, that is, the plant,
when self-pollinated, breeds true for the trait, then all you have to do
to continue obtaining that trait is to self the plant and its offspring.
Sometimes a trait cannot be fixed and many plants may have to be grown
in order to produce a few showing the desired trait.
|If red has incomplete dominance over yellow, then the seeds represented
in Fig. 6 will develop flowers having a color ratio of 1:2:1 - out of every
four flowers, one will be red, two will be mauve, and one will be yellow.
Continuing to use Fig. 6 as an example, let us now
assume that you wish to fix a trait by self-pollinating the plants of the
F2 generation. We will assume three different situations: (1)
you wish to breed only yellow flowers; (2) you wish to breed only red flowers;
and (3) you wish to breed only mauve flowers.
(1) Breeding for yellow. As explained above and illustrated in
Figs. 7 and 8, regardless of whether
there is complete or incomplete dominance, you may expect one out of every
four flowers to be yellow (rr). These yellow flowers are pure, that is,
they have no R genes. Therefore if you self them, they and their offspring
will breed true for yellow (Fig. 9).
(2) Breeding for red. If red is the dominant gene, the F2
generation will produce three red flowers out of four, some of the red
flowers containing Rr and some containing RR (see Fig.
7). The problem, then, is to isolate the ones containing RR, since
they are the ones that will breed true. If you self all the plants with
red flowers, some will produce offspring (the F3 generation)
with all red flowers and some will produce offspring with some red and
some yellow flowers. The self-pollinated parents that produced both red
and yellow offspring can be discarded -- they obviously are the ones containing
Rr. But the parents that produced only red offspring have bred true --
they evidently contain RR. To keep track of which red flowers produce only
red-flowered offspring, it is important to keep the seed from each self
separate and to keep records. Once you have isolated
the red flowers that produce only red flowers in their offspring, then
all you need to do to continue obtaining red flowers is to continue selfing
the parents or the offspring.
||When a purebred plant is self-pollinated, it breeds true; that is,
the offspring are all identical to the parent and to each other. нн
If red has incomplete dominance, only one out of four flowers will be
red (see Fig. 8), and since it is pure (containing
RR) you'll have no trouble in fixing the trait.
(3) Breeding for mauve. As explained earlier, mauve flowers will
be obtained if red has incomplete dominance over yellow. In the F2
generation, if there is incomplete dominance, two out of every four flowers
will be mauve (see Fig. 8). When you self these flowers,
segregation will again occur and you will obtain the same ratio as occurred
in the F2 generation -- 1:2:1. In other words, the color mauve
cannot be fixed. The most you can expect in the long run, if you self mauve
flowers, is two mauve flowers out of every four.
Breeding for other traits. The above discussion is given only
as an example to explain the process of heredity. By crossing and selfing
plants and observing the ratios of the traits that you are interested in,
you can soon find out whether you are dealing with complete or incomplete
dominance. (A 3:1 ratio in the F2 generation usually indicates
complete dominance; a 1:2:1 ratio, incomplete dominance.) You will then
be able to tell whether or not it is possible to fix a trait.
You may wish to breed for more than one trait. Some traits that are
often relatively easy to change are: yield, flower color, plant size, leaf
size, fruit shape, and fruit size. With two genes for each trait in each
parent, you will have to grow many offspring before one will appear with
all the desired traits. Even if you are interested in replacing only one
trait with a more desirable one, you may have to grow many plants in order
to obtain one that shows the desired trait without showing any changes
in other traits. Some characteristics, such as yield and fruit size, are
governed by many genes, and vast numbers of plants may be needed to obtain
the desired result.
Hybrids. F1 hybrids of vegetables and flowers are
often mentioned in seed catalogues and breeding programs. These varieties
are the result of crosses between two pure lines. The purpose of the cross
is to bring together in a plant desirable traits possessed separately by
the parent plants. Some characteristics obtained in this way are increased
vigor, uniformity, and earliness.
Seed from the hybrid will produce many plants unlike the hybrid. Therefore,
in order to maintain all the desirable characteristics of the hybrid, the
original parents are crossed each year. It is seldom worthwhile to save
seed from the hybrid for commercial production, although it can be used
in a breeding program.
General Breeding Techniques
The equipment required for plant breeding is relatively inexpensive and
easy to use. Here are some items that you may find useful:
Magnifying glass (10 or 15 power)
Small sharp-pointed scissors
Small containers or vials
Rubber bands or soft wire
Paper or cellophane bags
When to breed
Prepollination steps generally should begin just before the flower opens.
If you wait until after it opens, it may be pollinated by natural means,
which would make the flower useless for experimentation. Since most plants
bloom over a period of time, even if some of the flowers on a plant have
bloomed before you have prepared them for breeding, there will probably
be others that haven't opened yet and that you can still experiment with.
Extremely high temperatures or moist conditions are harmful to pollen.
For best results you should pollinate plants on dry days during the cool
hours of the morning or as soon as the anthers have split open.
Selecting the parents
Some plants have natural barriers to cross- or self-pollination. It is
advisable to check for this before you begin breeding, for although you
frequently can overcome these barriers, some plants cannot be artificially
pollinated. An example of a barrier that can be overcome is the natural
cross-pollination prohibitor of snapdragons; snapdragon flowers are constructed
so as to prevent entry of wind-borne pollen from other flowers. It is possible,
however, to open the flower by hand. An example of a barrier that cannot
be overcome is the self-pollination prohibitor of some orchids; the stigmas
of certain orchids produce a substance which kills the pollen of flowers
of the same plant. The mechanism that performs this cannot be removed without
destroying the pistil.
The plants you select for breeding should be sturdy and healthy. It
is usually easier to tell which ones are healthy after a few flowers on
the plant have bloomed.
In choosing a pollen parent, select one that has a heavy yellow powder
on the anther. This powder is the pollen. If you brush your fingernail
against the anther, a trace of pollen should adhere to your nail. A fresh
flower is more likely to have healthier pollen than one that has started
to wilt or dry out.
In choosing a seed parent, examine the stigma. It should have either
a glistening substance on it that is sticky to the touch or a "hairy" surface.
It is this substance or surface that retains the pollen, thus making fertilization
Once you have selected the pollen and seed parents, you are ready to
The first step is to mark those flowers that are to serve as pollen parents
and those that are to serve as seed parents. This can be done with colored
thread, one color for the male and another color for the female. Or you
can use paper labels, covered with varnish to protect them from the weather.
Some plant breeders use bands designed for marking chickens.
Your next step is to protect the plant from unwanted pollen. If the plant
is to be cross-pollinated, the stamens will have to be removed to prevent
the possibility of selfing. The removal of the stamen is called emasculation.
It should be performed before the anthers split open to release pollen.
This may require opening the flower by hand before it is ready to bloom.
Emasculation can be accomplished by: (1) pinching off the stamens or anthers
with tweezers (Fig. 10), or (2) snipping off the stamens or anthers with
sharp-pointed scissors, or (3) removing the petals to which the stamens
are sometimes attached. A magnifying glass will be particularly useful
||Emasculation is necessary to prevent self-pollination. Remove all
anthers or stamens. нн
Both the seed and pollen parents should be protected from contamination
by foreign pollen. This can be done by one of the following methods:
Closing the flower. In many flowers, such as morning-glories,
petunias, and lilies, the petals can be closed around the floral organs
with a piece of soft wire, string, or rubber band (Fig. 11) Care should
be taken not to tear the petals.
Covering the flower. Some flowers, such as composite flowers,
cannot be closed. To protect them from unwanted pollen, you can cover the
flower with a paper bag. Or, if you wish to observe the flower at all times,
you can cover it with a cellophane bag. The bag should be held securely
in place with a paper clip or string (Fig. 11).
Flowers that are to be self-pollinated should likewise be protected from
foreign pollen by either closing or covering the flower.
||To protect the flower from unwanted pollen, close it with a string
or soft wire (left) or cover it with a bag (right). A cellophane or plastic
bag will permit you to observe the flower at all times. нн
If the plant is grown indoors there is little likelihood of contamination
by foreign pollen and you do not have to cover or close the flower. However,
indoor as well as outdoor plants require emasculation to avoid self- pollination.
Crossing. There are several methods that can be used for crosspollinating
flowers. Here are four of the most common methods:
Each time you use different pollen, be sure to first wash with alcohol
the camel-hair brush, tweezers, and any other item which might have touched
some pollen. This step is very important to prevent pollination of the
seed parent with unwanted pollen that has adhered to the equipment. After
you wash the instruments be sure that they are dry before using them again.
Remove the stamens from the pollen parent with tweezers. Place the stamens
in a small container. Remove the protector from the seed parent. Holding
a stamen with the tweezers, gently brush the anther across the stigma (Fig.
12). Replace the protector.
||One way to cross-pollinate is to remove the stamens from the pollen
parent and then, holding a stamen with tweezers, brush the anther across
the stigma of the seed parent. нн
Cut the flower that is serving as the pollen parent. Remove the protector
from the seed parent. With tweezers remove a stamen from the pollen parent
and brush an anther gently across the stigma of the seed parent. Replace
With a camel-hair brush, transfer the pollen from the anthers of the pollen
parent into a small container. Remove the protector from the seed parent
and brush the pollen across the stigma (Fig. 13). Replace the protector.
||If the stamens are too small or too difficult to grasp, you can
cross-pollinate by transferring the pollen from the anthers with a brush
into a container and then brushing the pollen onto the stigma. нн
Shake the bagged pollen parent so that the pollen is collected in the bag
that is covering it for protection. Remove the bag from the pollen parent,
being careful not to spill the pollen. Remove the protector from the seed
parent and place the bag containing the pollen over the seed parent and
shake the bag so that pollen falls on the stigmatic surfaces. This is usually
done on corn.
Selfing. Procedures for self-pollinating flowers will depend
on the type of flower. For perfect flowers, your job is done once you have
closed or covered the flower, although you can sometimes help the pollen
land on the stigma by shaking the flower once a day for several days after
the pollen develops.
Only those composite flowers containing both disc and ray florets can
be self-pollinated. Since they have both pistils and stamens, they can
be selfed in the same way as perfect flowers.
With imperfect flowers, you will be able to self only those flowers
that are on the same plant. In selfing imperfect flowers, the pollen from
the staminate flower must be transferred to the stigma of the pistillate
flower on the same plant. To do this you can use any of the methods given
above for cross-pollination.
Immediately after pollination, close or cover the flower again. The next
step is to label the seed parent. The standard method of labeling is as
||Each pollinated flower should bear a label containing the name of
the seed parent, the letter X (to signify a cross), the name of the pollen
parent, and the date of the cross. нн
Once the seed parent is labeled, your next step is to record the cross
or self in a notebook. Keeping complete and accurate records of your breeding
operations is very important. The information that you record should contain
all essential facts regarding the cross or self so that you can refer to
it at a later time and even do the job again from the beginning if necessary.
Write on the label in the following order: (a) the number that you have
assigned or the variety name of the seed parent; (b) the letter X; (c)
the number or variety name of the pollen parent; and (d) the date the cross
was made (Fig. 14).
Attach the label to the stem just below the flower that has been pollinated.
A separate form or page should be used for each cross or self that you
make. An easy way to keep track of the offspring is to assign consecutive
numbers to each generation resulting from each cross or self.н
Kind of plant____________________________________________
ннннннннннннннннннннннн (name or number)
ннннннннннннннннннннннн (name or number)
Date cross made _________ Number to be assigned offspring __________
Traits of seed parent__________________________________________
Traits of pollen parent________________________________________
Traits of desired
Date of first flowering
Date of pollination
no. of seed
Observing the outcome
Once pollination has been completed, a period must elapse during which
fertilization and subsequent development of the fertilized ovule take place.
When the seed is completely developed, it should be harvested. In plants
grown for flowers, harvest the seed as soon as the seed pod is dry or when
it just starts to split open. In fruits and vegetables the seed will be
fully developed and ready for harvesting when the seed-bearing parts have
reached maturity. (For more information on harvesting seed, see
As you harvest the seed, place them in packets bearing the assigned
offspring number. Be careful to keep seed from different crosses and selfs
in separate packets. Air-dry the seed in a fairly warm spot for a week
or so and then store them in a cool, dry place.
As soon as practicable, the seed should be planted and the results entered
in your record book. The offspring can then be used for further study.
Any seeds that are not planted should be saved in case of crop failure.
An important point to remember is that if both parents are pure, segregation
usually will not become apparent until the F2 generation, and
you may have to breed several generations before you can obtain a plant
that will breed true.
Breeding Experiments for You to Try
The following crops have been selected to serve as examples of the various
types of flowers encountered in plant breeding -- perfect, imperfect, and
composite. The techniques suggested here, with slight modifications, can
be used for crossing other plants having similar flowering systems.
Corn is a good plant for the amateur plant breeder to experiment with because
it is easy to work with and often shows visible changes in the kernels
of the F1 generation. Some of the varieties that you can work
with are yellow field corn, white field corn, sweet corn, popcorn, calico
corn, and strawberry corn.
Corn has an imperfect flower, with both staminate and pistillate flowers
on the same plant (Fig. 15). The staminate flower is the tassel; the pistillate
flower is the ear. The silk, which has a hairy surface throughout most
of its length that is receptive to pollen, is the corn's stigma.
||Corn has an imperfect flower. The staminate flower is the tassel
and the pistillate is the ear. нн
Decide on the varieties of corn that you wish to cross and plant the seeds
in your experimental area. You can work with as many varieties as you wish,
but if this is your first experiment it will be best to limit yourself
to three or four. Corn tassels lose their pollen in a relatively short
time, so in order to have the pollen of the different varieties ready at
about the same time, plant a few seeds of each variety weekly over a period
of weeks. Mark the rows for future reference.
As soon as the plants develop ear shoots and before the silks emerge, cover
the shoots with a loosely fitted bag secured at the bottom with a paper
clip or string. A clear plastic or glassine bag will allow you to watch
the silks develop. Cover several ear shoots, for the more that are pollinated
the greater your chances of getting desired results.
Keep a close watch on the ears. When the silks are visible the ear is ready
for pollination. The appropriate tassel should then be bagged for use the
next morning. The bag should be secured tightly at the base of the tassel
to keep the pollen from falling out and to keep it from becoming contaminated
with other pollen.
The following morning shake the bagged tassel vigorously to loosen the
pollen. Remove the bag from the tassel, bending the tassel downward to
prevent pollen from spilling out (Fig. 16).
Remove the bag covering the silk. Place the bag from the tassel over the
ear, being careful not to spill any pollen. Secure the bag to the stalk
below the ear and shake (Fig. 17).
||When collecting corn pollen, bend the tassel downward to prevent
the pollen from spilling out of the bag. нн
||Place the bag containing pollen on the ear of the corn, secure the
bag to the stalk below the ear, and label the pollinated ear. нн
Label each of the ears that have been pollinated with the name or number
of the plants that are serving as seed and pollen parents and enter the
cross in your record book.
Leave the bagged ear on the plant until the leaves dry. After the pollinated
ear has dried, record any changes that may have occurred in the kernels.
Then harvest the kernels and save them for future planting.
The flowers of squash are large and easy to work with. The flowers are
imperfect, with both the male and female flowers on the same plant. The
pistillate flower is enlarged at the base while the staminate flower is
borne on a long stem (see Fig. 2).
Although some varieties of squash are cross-sterile, that is, they cannot
be crossed, many others can be crossed. It is also possible to cross some
varieties of pumpkin and squash.
Remember that if you are crossing pure varieties, segregation won't
occur until the F2 generation; that is, the various colors and
shapes that the offspring have inherited will not all show up until the
offspring are self-pollinated and the seeds that they produce bear fruit.
Plant the different varieties of squash. Label the rows so that you will
know which plants are to be seed parents and which are to be pollen parents.
You won't have any difficulty in having the male and female parents ready
for pollination at the same time, for squash produces flowers over a relatively
The evening before the flowers open, place a bag over them or close the
flowers with rubber bands or string (see Fig. 11).
It is easy to tell in the evening which flowers will open the following
morning. The flowers will be slightly open, revealing the inner color,
which is brighter than the color on the outside of the petals. Both pistillate
and staminate flowers should be protected, to prevent bees from entering.
Early in the morning on the day that the flowers open, remove the stamens
from the staminate flower and gently rub the anthers across the stigma
of the pistillate flower. (Fig. 18.)
||To pollinate squash, gently rub the anther across the stigma of
the pistillate flower. нн
Replace the bag on the pistillate flower.
Label the pollinated pistillate flower and, enter the cross in your record
You can remove the protective covering when the petals have dried on the
pistillate flower or when the ovary bulges through the bag.
Harvest the seeds when the fruit is mature (Fig. 19). Maturity is usually
reached about 45 days after pollination. Save the seeds until the next
year, when you will (1) plant them, (2) self-pollinate the resulting flowers,
and (3) again harvest the seeds. The year after that, when you plant the
seeds resulting from the self-pollination of the F generation, segregation
will probably occur.
||Harvest the squash seeds and save them for planting next year.
The tomato has a perfect flower, with male and female organs both within
the same blossom (Fig. 20). The sex organs are easy to tell apart and therefore
it is relatively easy to cross tomatoes.
||The reproductive parts of the tomato. нн
For this experiment, choose tomatoes with two different fruit colors,
such as red and yellow. If the tomatoes are pure, variations in color will
show up in the F2 generation. Along with color variations, there
will probably be changes in fruit shape and other plant characteristics.
Plant the seed or young plants and label the rows so that you will know
which will bear red fruit and which will bear yellow fruit. The tomato
produces flowers for several weeks, so there will be no difficulty in having
pollen and seed parents ready at the same time.
As soon as the flowers that will serve as seed parents begin to open, the
plant should be emasculated. The stamens are fused to the petals, so all
you need to do to emasculate the plant is to pull out the petals.
After you remove the stamens from the seed parents, cover the flowers securely.
The pollen parents that you wish to use should be in the same stage of
opening as the seed parents. Cover the pollen parents at the same time
that you cover the seed parents.
As soon as the pollen-parent flower is completely open, remove one of its
stamens with scissors or tweezers. Remove the bag from the seed parent
and gently brush the anther across the stigma.
Replace the bag that was removed from the seed-parent flower.
Label the flower that was pollinated (Fig. 21) and enter the cross in your
||A tomato flower that has been emasculated, pollinated, and labeled.
After a week remove the bag from the seed parent and leave it off.
When the fruit is ripe, harvest and store the seeds for future planting
and self-pollination (Fig. 22).
||Harvest the tomato seeds and save them for planting next year.
Chrysanthemums are composite flowers. If they are not available for crossing,
you can use any other composite flower, such as zinnias or asters. Composite
flowers are more difficult to cross than complete or incomplete flowers.
If you are crossing plants for the first time, it would probably be best
if you gained experience by first crossing a simpler flower, such as squash,
iris, or lily, before trying to cross composite flowers.
Plant the seeds or young plants. Mums, zinnias, and asters bloom over a
relatively long period, so there will be no difficulty in having pollen
and seed parents ready at the same time.
Immediately after the seed-parent flowers open and before the pollen is
mature, remove the disc florets with tweezers. As shown in Fig.
3, the disc florets are in the center of the flower. They are easy
to identify because they are often yellow in color and they have both pistils
and stamens, whereas the ray florets, which surround the disc florets,
have only pistils. Remove some of the ray florets near the center of the
flower just to be sure that you haven't missed any of the disc florets.
If any disc florets are left on the flower, the flower is likely to be
self-pollinated (Fig. 23).
||When emasculating chrysanthemums, make sure that you remove all
of the disc florets. нн
After removing the disc florets, cover the seed-parent flowers with bags.
As soon as the flowers of the pollen parents open, cover them with bags.
Pollination will be more effective if you trim the ray florets on the seed
parents to just above the stigmas. This step can be done at any time after
the disc florets are removed and before pollination takes place (Fig. 24).
||Trim the ray florets for more effective pollination. нн
The flower is ready for pollination about four or five days after it opens.
Shortly before you pollinate the flower, dip a camel-hair brush in alcohol
and allow it to dry.
Remove a flower from the pollen parent. With the camel-hair brush, gently
brush the disc florets of the pollen-parent flower. The pollen should adhere
to the brush. Then gently brush the trimmed ray florets of the seed-parent
flower (see Fig. 13). Each time you change pollen
parents dip the brush in alcohol and let it dry.
Replace the protective bags on the seed and pollen parents.
Label the pollinated flower and enter the cross in your record book.
Three or four days later, repeat step 7. Be sure to use the same pollen
parent that you used the first time. Replace the bag on the seed parent.
Remove the bag from the seed parent about a week after the final pollination.
After the seed head has dried, harvest and store the seed (Fig. 25).
||Each mature ray floret will have a seed. Harvest these seeds and
plant them next year. нн
For further information on plant breeding the following books may prove
Practical Plant Breeding, W. J. C. Lawrence, George Allen and Unwin
Ltd., London, 1951.
1937 Yearbook of Agriculture, U. S. Department of Agriculture.
Simple, Practical Hybridising for Beginners, D. Gourlay Thomas,
John Gifford, Ltd., London, 1957.
Plant Breeding for Everyone, John Y. Beaty, Charles T. Branford
Co., Boston, 1954.
The Genetics of Garden Plants, M. B. Crane and W. J. C. Lawrence,
Macmillan Co., London, 1952.
Handbook on Breeding Ornamental Plants, Brooklyn Botanic Garden,
Brooklyn, N. Y., 1959.
Plant Breeding and Cytogenetics, Fred C. Elliott, McGraw-Hill Book
Co., New York, 1958.
Methods of Plant Breeding, H. K. Hayes, F. R. Immer, and D. C. Smith,
McGraw-Hill Book Company, New York, 1955.
General Genetics, Adrian M. Srb and Ray D. Owen, W. H. Freeman and
Co., San Francisco, 1952.
Vegetable and Flower Seed Production, Leslie R. Hawthorn and Leonard
H. Pollard, The Blakiston Co., New York, 1954.
Vegetable Growing, James S. Shoemaker, John Wiley and Sons, New
York, 1947. (Chapter 1 contains information on harvesting seeds.)
Explanation of Terms
The part of the stamen that develops and bears pollen.
A cluster of small flowers joined together in a flower head, having either
pistils only or both pistils and stamens.
Transfer of pollen from the anther of a flower to the stigma of a flower
of another plant.
small flower in a composite flower, having stamens and a pistil. Emasculation:
Removal of stamens or anthers to prevent selfpollination.
F1, F2, etc.:
Designation of generations of plants. F1 is the first filial
generation, F2 the second, etc.
The unit that carries hereditary traits.
The reproductive cell which, when united with a germ cell of the opposite
sex, develops into a new individual.
A flower having stamens or pistil, but not both.
The enlarged base of the pistil in which the seed develops.
A flower having both stamens and a pistil.
The female reproductive organ, which produces female germ cells and bears
the seed. Usually a pistil has an ovary, a style, and a stigma.
A small flower in a composite flower, having a pistil but no stamens.
Transfer of pollen from the anther of a flower to the stigma of the same
flower or of another flower on the same plant.
The male reproductive organ, which produces male germ cells, carried in
pollen grains. Usually a stamen has a filament and an anther.
The surface of the pistil on which pollen grains are deposited in the process
The usually elongated portion of the pistil connecting the stigma and ovary.
This circular was prepared by J. D. BUTLER, Assistant Professor
of Horticulture, and N. F. OEBKER, formerly Associate Professor of Horticulture.
The authors express appreciation to the following persons for their help
in preparing this circular: D. F. DAYTON, Professor of Plant Breeding;
G. M. FOSLER, Instructor in Floriculture; and R. W. HEPLER, formerly Research
Associate in Horticulture.