[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.]