_A field trip_:--Object: to collect common insects and study
their general characteristics; to study the food and shelter
relation of plant and insects. The pollination of flowers
should also be carefully studied so as to give the pupil a
general viewpoint as an introduction to the study of
biology.
_Laboratory exercise._--Examination of simple insect,
identification of parts--drawing. Examination and
identification of some orders of insects.
_Laboratory demonstration._--Life history of monarch and
some other butterflies or moths.
_Laboratory exercise._--Study of simple flower--emphasis on
work of essential organs, drawing.
_Laboratory exercise._--Study of mutual adaptations in a
given insect and a given flower, _e.g._ butter and eggs and
bumble bee.
_Demonstration of examples of insect pollination._
The Object of a Field Trip.--Many of us live in the city, where the crowded
streets, the closely packed apartments, and the city playgrounds form our
environment. It is very artificial at best. To understand better the
_normal environment_ of plants or animals we should go into the country.
Failing in this, an overgrown city lot or a park will give us much more
closely the environment as it touches some animals lower than man. We must
then remember that in learning something of the natural environment of
other living creatures we may better understand our own environment and our
relation to it.
On any bright warm day in the fall we will find insects swarming everywhere
in any vacant lot or the less cultivated parts of a city park.
Grasshoppers, butterflies alighting now and then on the flowers, brightly
marked hornets, bees busily working over the purple asters or golden rod,
and many other forms hidden away on the leaves or stems of plants may be
seen. If we were to select for observation some partially decayed tree, we
would find it also inhabited. Beetles would be found boring through its
bark and wood, while caterpillars (the young stages of butterflies or
moths) are feeding on its leaves or building homes in its branches.
Everywhere above, on, and under ground may be noticed small forms of life,
many of them insects. Let us first see how we would go to work to identify
some of the common forms we would be likely to find on plants. Then a
little later we will find out what they are doing on these plants.
[Illustration: An insect viewed from the side. Notice the head, thorax, and
abdomen. What other characters do you find?]
How to tell an Insect.--A bee is a good example of the group of animals we
call _insects_. If we examine its body carefully, we notice that it has
three regions, a front part or _head_, a middle part called the _thorax_,
and a hind portion, jointed and hairy, the _abdomen_. We cannot escape
noting the fact that this insect has wings with which it flies and that it
also has legs. The three pairs of legs, which are jointed and provided with
tiny hooks at the end, are attached to the thorax. Two pairs of delicate
wings are attached to the upper or _dorsal_ side of the thorax. The thorax
and indeed the entire body, is covered with a hard shell of material
similar to a cow's horn, there being no skeleton inside for the attachment
of muscles. If we carefully watch the abdomen of a living bee, we notice it
move up and down quite regularly. The animal is breathing through tiny
breathing holes called _spiracles_, placed along the side of the thorax and
abdomen. Bees also have compound eyes. Wings are not found on all insects,
but all the other characters just given are marks of the great group of
animals we call _insects_.
[Illustration: Part of the compound eye of an insect (highly magnified).]
Forms to be looked for on a Field Trip.--Inasmuch as there are over 360,000
different species or kinds of insects, it is evident that it would be a
hopeless task for us even to think of recognizing all of them. But we can
learn to recognize a few examples of the common forms that might be met on
a field trip. In the fields, on grass, or on flowering plants we may count
on finding members from six groups or _orders_ of insects. These may be
known by the following characters.
The order _Hymenoptera_ (membrane wing) to which the bees, wasps, and ants
belong is the only insect group the members of which are provided with true
stings. This sting is placed in a sheath at the extreme hind end of the
abdomen. Other characteristics, which show them to be insects, have been
given above.
Butterflies or moths will be found hovering over flowers. They belong to
the order _Lepidoptera_ (scale wings). This name is given to them because
their wings are covered with tiny scales, which fit into little sockets on
the wing much as shingles are placed on a roof. The dust which comes off on
the fingers when one catches a butterfly is composed of these scales. The
wings are always large and usually brightly colored, the legs small, and
one pair is often inconspicuous. These insects may be seen to take liquid
food through a long tubelike organ, called the _proboscis_, which they keep
rolled up under the head when not in use. The young of the butterfly or
moth are known as _caterpillars_ and feed on plants by means of a pair of
hard jaws.
Grasshoppers, found almost everywhere, and crickets, black grasshopper-like
insects often found under stones, belong to the order _Orthoptera_
(straight wings). Members of this group may usually be distinguished by
their strong, jumping hind legs, by their chewing or biting mouth parts,
and by the fact that the hind wings are folded up under the somewhat
stiffer front wings.
[Illustration: Forms of life to be met on a field trip. _A_, The red-legged
locust, one of the _Orthoptera_; _o_, the egg-layer, about natural size.
_B_, the honey bee, one of the _Hymenoptera_, about natural size. _C_, a
bug, one of the _Hemiptera_, about natural size. _D_, a butterfly, an
example of the _Lepidoptera_, slightly reduced. _E_, a house fly, an
example of the _Diptera_, about twice natural size. _F_, an orb-weaving
spider, about half natural size. (This is not an insect, note the number of
legs.) _G_, a beetle, slightly reduced, one of the _Coleoptera_.]
Another group of insects sometimes found on flowers in the fall are flies.
They belong to the order _Diptera_ (two wings). These insects are usually
rather small and have a single pair of gauzy wings. Flies are of much
importance to man because certain of their number are disease carriers.
Bugs, members of the order _Hemiptera_ (half wings), have a jointed
proboscis which points backward between the front legs. They are usually
small and may or may not have wings.
The beetles or _Coleoptera_ (sheath wings), often mistaken for bugs by the
uneducated, have the first pair of hardened wings meeting in a straight
line in the middle of the back, the second pair of wings being covered by
them. Beetles are frequently found on goldenrod blossoms in the fall.
Other forms of life, especially _spiders_, which have four pairs of walking
legs, _centipedes_ and _millepedes_, both of which are wormlike and have
many pairs of legs, may be found.
Try to discover members of the six different orders named above. Collect
specimens and bring them to the laboratory for identification.
Why do Insects live on Plants?--We have found insect life abundant on
living green plants, some visiting flowers, others hidden away on the
stalks or leaves of the plants. Let us next try to find out _why_ insects
live among and upon flowering green plants.
The Life History of the Milkweed Butterfly.--If it is possible to find on
our trip some growing milkweed, we are quite likely to find hovering near,
a golden brown and black butterfly, the monarch or milkweed butterfly
(_Anosia plexippus_). Its body, as in all insects, is composed of three
regions. The monarch frequents the milkweed in order to lay eggs there.
This she may be found doing at almost any time from June until September.
Egg and Larva.--The eggs, tiny hat-shaped dots a twentieth of an inch in
length, are fastened singly to the underside of milkweed leaves. Some
wonderful instinct leads the animal to deposit the eggs on the milkweed,
for the young feed upon no other plant. The eggs hatch out in four or five
days into rapid-growing wormlike caterpillars, each of which will shed its
skin several times before it becomes full size. These caterpillars possess,
in addition to the three pairs of true legs, additional pairs of _prolegs_
or caterpillar legs. The animal at this stage is known as a _larva_.
Formation of Pupa.--After a life of a few weeks at most, the caterpillar
stops eating and begins to spin a tiny mat of silk upon a leaf or stem. It
attaches itself to this web by the last pair of prolegs, and there hangs in
the dormant stage known as the _chrysalis_ or _pupa_. This is a resting
stage during which the body changes from a caterpillar to a butterfly.
[Illustration: Monarch butterfly: adults, larvae, and pupa on their food
plant, the milkweed. (From a photograph loaned by the American Museum of
Natural History.)]
The Adult.--After a week or more of inactivity in the pupa state, the outer
skin is split along the back, and the adult butterfly emerges. At first the
wings are soft and much smaller than in the adult. Within fifteen minutes
to half an hour after the butterfly emerges, however, the wings are
full-sized, having been pumped full of blood and air, and the little insect
is ready after her wedding flight to follow her instinct to deposit her
eggs on a milkweed plant.
Plants furnish Insects with Food.--Food is the most important factor of any
animal's environment. The insects which we have seen on our field trip feed
on the green plants among which they live. Each insect has its own
particular favorite food plant or plants, and in many cases the eggs of the
insect are laid on the food plant so that the young may have food close at
hand. Some insects prefer the rotted wood of trees. An American zoologist,
Packard, has estimated that over 450 kinds of insects live upon oak trees
alone. Everywhere animals are engaged in taking their nourishment from
plants, and millions of dollars of damage is done every year to gardens,
fruits, and cereal crops by insects.
[Illustration: Damage done by insects. These trees have been killed by
boring insects.]
All Animals depend on Green Plants.--But insects in their turn are the food
of birds; cats and dogs may kill birds; lions or tigers live on still
larger defenseless animals as deer or cattle. And finally comes man, who
eats the bodies of both plants and animals. But if we reduce this search
after food to its final limit, we see that green plants provide _all_ the
food for animals. For the lion or tiger eats the deer which feeds upon
grass or green shoots of young trees, or the cat eats the bird that lives
on weed seeds. Green plants supply the food of the world. Later by
experiment we will prove this.
Homes and Shelter.--After a field trip no one can escape the knowledge that
plants often give animals a home. The grass shelters millions of
grasshoppers and countless hordes of other small insects which can be
obtained by sweeping through the grass with an insect net. Some insects
build their homes in the trees or bushes on which they feed, while others
tunnel through the wood, making homes there. Spiders build webs on plants,
often using the leaves for shelter. Birds nest in trees, and many other
wild animals use the forest as their home. Man has come to use all kinds of
plant products to aid him in making his home, wood and various fibers being
the most important of these.
What do Animals do for Plants?--So far it has seemed that green plants
benefit animals and receive nothing in return. We will later see that
plants and animals _together_ form a balance of life on the earth and that
one is necessary for the other. Certain substances found in the body wastes
from animals are necessary to the life of a green plant.
Insects and Flowers.--Certain other problems can be worked out in the fall
of the year. One of these is the biological interrelations between insects
and flowers. It is easy on a field trip to find insects lighting upon
flowers. They evidently have a reason for doing this. To find out why they
go there and what they do when there, it will be first necessary for us to
study flowers with the idea of finding out what the insects get from them,
and what the flowers get from the insects.
[Illustration: A section of a flower, cut lengthwise. In the center find
the pistil with the ovary containing a number of ovules. Around this organ
notice a circle of stalked structures, the stamens; the knobs at the end
contain pollen. The outer circles of parts are called the petals and
sepals, as we go from the inside outward.]
The Use and Structure of a Flower.--It is a matter of common knowledge that
flowers form fruits and that fruits contain seeds. They are, then, very
important parts of certain plants. Our field trip shows us that flowers are
of various shapes, colors, and sizes. It will now be our problem first to
learn to know the parts of a flower, and then find out how they are fitted
to attract and receive insect visitors.
The Floral Envelope.--In a flower the expanded portion of the flower stalk,
which holds the parts of the flower, is called the _receptacle_. _The green
leaflike parts covering the unopened flower are called the sepals._
Together they form the _calyx_.
_The more brightly colored structures are the petals._ Together they form
the _corolla_. The corolla is of importance, as we shall see later, in
making the flower conspicuous. Frequently the petals or corolla have bright
marks or dots which lead down to the base of the cup of the flower, where a
sweet fluid called _nectar_ is made and secreted. It is principally this
food substance, later made into honey by bees, that makes flowers
attractive to insects.
The Essential Organs.--A flower, however, could live without sepals or
petals and still do the work for which it exists. Certain _essential
organs_ of the flower are within the so-called floral envelope. They
consist of the _stamens_ and _pistil_, the latter being in the center of
the flower. The structures with the knobbed ends are called _stamens_. In a
single stamen the boxlike part at the end is the _anther_; the stalk which
holds the anther is called the _filament_. The anther is in reality a
hollow box which produces a large number of little grains called _pollen_.
Each pistil is composed of a rather stout base called the _ovary_, and a
more or less lengthened portion rising from the ovary called the _style_.
The upper end of the style, which in some cases is somewhat broadened, is
called the _stigma_. The free end of the stigma usually secretes a sweet
fluid in which grains of pollen from flowers of the same kind can grow.
Insects as Pollinating Agents.--Insects often visit flowers to obtain
pollen as well as nectar. In so doing they may transfer some of the pollen
from one flower to another of the same kind. This transfer of pollen,
called _pollination_, is of the greatest use to the plant, as we will later
prove. No one who sees a hive of bees with their wonderful communal life
can fail to see that these insects play a great part in the life of the
flowers near the hive. A famous observer named Sir John Lubbock tested bees
and wasps to see how many trips they made daily from their homes to the
flowers, and found that the wasp went out on 116 visits during a working
day of 16 hours, while the bee made but a few less visits, and worked only
a little less time than the wasp worked. It is evident that in the course
of so many trips to the fields a bee must light on hundreds of flowers.
[Illustration: Bumblebees. _a_, queen; _b_, worker; _c_, drone.]
Adaptations in a Bee.--If we look closely at the bee, we find the body and
legs more or less covered with tiny hairs; especially are these hairs found
on the legs. _When a plant or animal structure is fitted to do a certain
kind of work, we say it is adapted to do that work._ The joints in the leg
of the bee adapt it for complicated movements; the arrangement of stiff
hairs along the edge of a concavity in one of the joints of the leg forms a
structure well adapted to hold pollen. In this way pollen is collected by
the bee and taken to the hive to be used as food. But while gathering
pollen for itself, the dust is caught on the hairs and other projections on
the body or legs and is thus carried from flower to flower. The value of
this to a flower we will see later.
Field Work.--Is Color or Odor in a Flower an Attraction to an Insect?--Sir
John Lubbock tried an experiment which it would pay a number of careful
pupils to repeat. He placed a few drops of honey on glass slips and placed
them over papers of various colors. In this way he found that the honeybee,
for example, could evidently distinguish different colors. Bees seemed to
prefer blue to any other color. Flowers of a yellow or flesh color were
preferred by flies. It would be of considerable interest for some student
to work out this problem with our native bees and with other insects by
using paper flowers and honey or sirup. Test the keenness of sight in
insects by placing a white object (a white golf ball will do) in the grass
and see how many insects will alight on it. Try to work out some method by
which you can decide whether a given insect is attracted to a flower by
odor alone.
The Sight of the Bumblebee.--The large eyes located on the sides of the
head are made up of a large number of little units, each of which is
considered to be a very simple eye. The large eyes are therefore called the
_compound eyes_. All insects are provided with compound eyes, with simple
eyes, or in most cases with both. The simple eyes of the bee may be found
by a careful observer between and above the compound eyes.
Insects can, as we have already learned, distinguish differences in color
at some distance; they can see _moving_ objects, but they do not seem to be
able to make out form well. To make up for this, they appear to have an
extremely well-developed sense of smell. Insects can distinguish at a great
distance odors which to the human nose are indistinguishable. Night-flying
insects, especially, find the flowers by the odor rather than by color.
[Illustration: The head of a bee. _A_, antennae or "feelers"; _E_, compound
eye; _S_, simple eye; _M_, mouth parts; _T_, tongue.]
Mouth Parts of the Bee.--The mouth of the bee is adapted to take in the
foods we have mentioned, and is used for the purposes for which man would
use the hands and fingers. The honeybee laps or sucks nectar from flowers,
it chews the pollen, and it uses part of the mouth as a trowel in making
the honeycomb. The uses of the mouth parts may be made out by watching a
bee on a well-opened flower.
Suggestions for Field Work.--In any locality where flowers are abundant,
try to answer the following questions: How many bees visit the
locality in ten minutes? How many other insects alight on the flowers?
Do bees visit flowers of the same kinds in succession, or fly from one
flower on a given plant to another on a plant of a different kind? If the
bee lights on a flower cluster, does it visit more than one flower in the
same cluster? How does a bee alight? Exactly what does the bee do
when it alights?
[Illustration: Flower cluster of "butter and eggs."]
Butter and Eggs (_Linaria vulgaris_).--From July to October this very
abundant weed may be found especially along roadsides and in sunny fields.
The flower cluster forms a tall and conspicuous cluster of orange and
yellow flowers.
The corolla projects into a spur on the lower side; an upper two-parted lip
shuts down upon a lower three-parted lip. The four stamens are in pairs,
two long and two short.
[Illustration: Diagram to show how the bee pollinates "butter and eggs."
The bumblebee, upon entering the flower, rubs its head against the long
pair of anthers (_a_), then continuing to press into the flower so as to
reach the nectar at (_N_) it brushes against the stigma (_S_), thus
pollinating the flower. Inasmuch as bees visit other flowers in the same
cluster, cross-pollination would also be likely. Why?]
Certain parts of the corolla are more brightly colored than the rest of the
flower. This color is a guide to insects. Butter and eggs is visited most
by bumblebees, which are guided by the orange lip to alight just where they
can push their way into the flower. The bee, seeking the nectar secreted in
the spur, brushes his head and shoulders against the stamens. He may then,
as he pushes down after nectar, leave some pollen upon the pistil, thus
assisting in _self-pollination_. Visiting another flower of the cluster, it
would be an easy matter accidentally to transfer this pollen to the stigma
of another flower. In this way pollen is carried by the insect to another
flower of the same kind. This is known as _cross-pollination_. _By
pollination we mean the transfer of pollen from an anther to the stigma of
a flower. Self-pollination is the transfer of pollen from the anther to the
stigma of the same flower; cross-pollination is the transfer of pollen from
the anthers of one flower to the stigma of another flower on the same or
another plant of the same kind._
[Illustration: A wild orchid, a flower of the type from which Charles
Darwin worked out his theory of cross-pollination by insects.]
History of the Discoveries regarding Pollination of Flowers.--Although the
ancient Greek and Roman naturalists had some vague ideas on the subject of
pollination, it was not until the first part of the nineteenth century that
a book appeared in which a German named Conrad Sprengel worked out the
facts that the structure of certain flowers seemed to be adapted to the
visits of insects. Certain facilities were offered to an insect in the way
of easy foothold, sweet odor, and especially food in the shape of pollen
and nectar, the latter a sweet-tasting substance manufactured by certain
parts of the flower known as the nectar glands. Sprengel further discovered
the fact that pollen could be and was carried by the insect visitors from
the anthers of the flower to its stigma. It was not until the middle of the
nineteenth century, however, that an Englishman, Charles Darwin, applied
Sprengel's discoveries on the relation of insects to flowers by his
investigations upon cross-pollination. The growth of the pollen on the
stigma of the flower results eventually in the production of seeds, and
thus new plants. Many species of flowers are self-pollinated and do not do
so well in seed production if cross-pollinated, but Charles Darwin found
that some flowers which were self-pollinated did not produce so many seeds,
and that the plants which grew from their seeds were smaller and weaker
than plants from seeds produced by cross-pollinated flowers of the same
kind. He also found that plants grown from cross-pollinated seeds tended to
_vary_ more than those grown from self-pollinated seed. This has an
important bearing, as we shall see later, in the production of new
varieties of plants. Microscopic examination of the stigma at the time of
pollination also shows that the pollen from another flower usually
germinates before the pollen which has fallen from the anthers of the same
flower. This latter fact alone in most cases renders it unlikely for a
flower to produce seeds by its own pollen. Darwin worked for years on the
pollination of many insect-visited flowers, and discovered in almost every
case that showy, sweet-scented, or otherwise attractive flowers were
adapted or fitted to be cross-pollinated by insects. He also found that, in
the case of flowers that were inconspicuous in appearance, often a
compensation appeared in the odor which rendered them attractive to certain
insects. The so-called carrion flowers, pollinated by flies, are examples,
the odor in this case being like decayed flesh. Other flowers open at
night, are white, and provided with a powerful scent. Thus they attract
night-flying moths and other insects.
Other Examples of Mutual Aid between Flowers and Insects.--Many other
examples of adaptations to secure cross-pollination by means of the visits
of insects might be given. The mountain laurel, which makes our hillsides
so beautiful in late spring, shows a remarkable adaptation in having the
anthers of the stamens caught in little pockets of the corolla. The weight
of the visiting insect on the corolla releases the anther from the pocket
in which it rests so that it springs up, dusting the body of the visitor
with pollen.
[Illustration: The condition of stamens and pistils on the spiked
loosestrife (_Lythrum salicaria_).]
In some flowers, as shown by the primroses or primula of our hothouses, the
stamens and pistils are of different lengths in different flowers. Short
styles and long or high-placed filaments are found in one flower, and long
styles with short or low-placed filaments in the other. Pollination will be
effected only when some of the pollen from a low-placed anther reaches the
stigma of a short-styled flower, or when the pollen from a high anther is
placed upon a long-styled pistil. There are, as in the case of the
loosestrife, flowers having pistils and stamens of three lengths. Pollen
only grows on pistils of the same length as the stamens from which it came.
The milkweed or butterfly weed already mentioned is another example of a
flower adapted to insect pollination.[1]
Footnote 1: For an excellent account of cross-pollination of this
flower, the reader is referred to W. C. Stevens, _Introduction to
Botany_. Orchids are well known to botanists as showing some very
wonderful adaptations. A classic easily read is Darwin, _On the
Fertilization of Orchids_.
[Illustration: The pronuba moth within the yucca flower.]
A very remarkable instance of insect help is found in the pollination of
the yucca, a semitropical lily which lives in deserts (to be seen in most
botanic gardens). In this flower the stigmatic surface is above the anther,
and the pollen is sticky and cannot be transferred except by insect aid.
This is accomplished in a remarkable manner. A little moth, called the
_pronuba_, after gathering pollen from an anther, deposits an egg in the
ovary of the pistil, and then rubs its load of pollen over the stigma of
the flower. The young hatch out and feed on the young seeds which have
grown because of the pollen placed on the stigma by the mother. The baby
caterpillars eat some of the developing seeds and later bore out of the
seed pod and escape to the ground, leaving the plant to develop the
remaining seeds without further molestation.
[Illustration: The pronuba pollinating the pistil of the yucca.]
The fig insect (_Blastophaga grossorum_) is another member of the insect
tribe that is of considerable economic importance. It is only in recent
years that the fruit growers of California have discovered that the
fertilization of the female flowers is brought about by a gallfly which
bores into the young fruit. By importing the gallflies it has been possible
to grow figs where for many years it was believed that the climate
prevented figs from ripening.
[Illustration: Pod of yucca showing where the young pronubas escaped.]
Other Flower Visitors.--Other insects besides those already mentioned are
pollen carriers for flowers. Among the most useful are moths and
butterflies. Projecting from each side of the head of a butterfly is a
fluffy structure, the palp. This collects and carries a large amount of
pollen, which is deposited upon the stigmas of other flowers when the
butterfly pushes its head down into the flower tube after nectar. The
scales and hairs on the wings, legs, and body also carry pollen.
[Illustration: A humming bird about to cross-pollinate a lily.]
Flies and some other insects are agents in cross-pollination. Humming birds
are also active agents in some flowers. Snails are said in rare instances
to carry pollen. Man and the domesticated animals undoubtedly frequently
pollinate flowers by brushing past them through the fields.
[Illustration: A cornfield showing staminate and pistillate flowers, the
latter having become grains of corn. An ear of corn is a bunch of ripened
fruits.]
Pollination by the Wind.--Not all flowers are dependent upon insects or
other animals for cross-pollination. Many of the earliest of spring flowers
appear almost before the insects do. Such flowers are dependent upon the
wind for carrying pollen from the stamens of one flower to the pistil of
another. Most of our common trees, oak, poplar, maple, and others, are
cross-pollinated almost entirely by the wind.
Flowers pollinated by the wind are generally inconspicuous and often lack a
corolla. The anthers are exposed to the wind and provided with much pollen,
while the surface of the stigma may be long and feathery. Such flowers may
also lack odor, nectar, and bright color. Can you tell why?
Imperfect Flowers.--Some flowers, the wind-pollinated ones in particular,
are imperfect; that is, they lack either stamens or pistils. Again, in some
cases, imperfect flowers having stamens only are alone found on one plant,
while those flowers having pistils only are found on another plant of the
same kind. In such flowers, cross-pollination must of necessity follow.
Many of our common trees are examples.
[Illustration: The flower of "Lady Washington" geranium, in which stamens
and pistil ripen at different times, thus insuring cross-pollination. _A_,
flower with ripe stamens; _B_, flower with stamens withered and ripe
pistil.]
Other Cases.--The stamens and pistil ripen at different times in some
flowers. The "Lady Washington" geranium, a common house plant, shows this
condition. Here also cross-pollination must take place if seeds are to be
formed.
Summary.--If we now collect our observations upon flowers with a view to
making a summary of the different devices flowers have assumed to prevent
self-pollination and to secure cross-pollination, we find that they are as
follows:--
_(1) The stamens and pistils may be found in separate flowers, either on
the same or on different plants._
_(2) The stamens may produce pollen before the pistil is ready to receive
it, or vice versa._
_(3) The stamens and pistils may be so placed with reference to each other
that pollination can be brought about only by outside assistance._
Artificial Cross-pollination and its Practical Benefits to Man.--Artificial
cross-pollination is practiced by plant breeders and can easily be tried in
the laboratory or at home. First the anthers must be carefully removed from
the bud of the flower so as to eliminate all possibility of
self-pollination. The flower must then be covered so as to prevent access
of pollen from without; when the ovary is sufficiently developed, pollen
from another flower, having the characters desired, is placed on the stigma
and the flower again covered to prevent any other pollen reaching the
flower. The seeds from this flower when planted _may_ give rise to plants
with the best characters of each of the plants which contributed to the
making of the seeds.