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Problem.—To discover the general interrelations of green plants and animals.

(a) Plants as homes for insects.

(b) Plants as food for insects.

(c) Insects as pollinating agents.

Laboratory Suggestions

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.

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.

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.

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.

Monarch butterfly: adults, larvæ, and pupa on their food plant, the milkweed. (From a photograph loaned by the American Museum of Natural History.)

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.

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 zoölogist, 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.

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.

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.

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.

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.

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.

The head of a bee. A, antennæ or "feelers"; E, compound eye; S, simple eye; M, mouth parts; T, tongue.

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.

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?

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.

Flower cluster of "butter and eggs."

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.

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.

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.

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]

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.

The pronuba pollinating the pistil of the yucca.

Pod of yucca showing where the young pronubas escaped.

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.

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.

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.

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.

A cornfield showing staminate and pistillate flowers, the latter having become grains of corn. An ear of corn is a bunch of ripened fruits.

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.

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.

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.

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.

[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.

Reference Books

elementary

Hunter, Laboratory Problems in Civic Biology. American Book Company.

Andrews, A Practical Course in Botany, pages 214–249. American Book Company.

Atkinson, First Studies of Plant Life, Chaps. XXV-XXVI. Ginn and Company.

Coulter, Plant Life and Plant Uses, pages 301–322. American Book Company.

Dana, Plants and their Children, pages 187–255. American Book Company.

Lubbock, Flowers, Fruits, and Leaves, Part I. The Macmillan Company.

Needham, General Biology, pages 1–50. The Comstalk Publishing Company.

Newell, A Reader in Botany, Part II, pages 1–96. Ginn and Company.

Sharpe, A Laboratory Manual in Biology, pages 43–48. American Book Company.

advanced

Bailey, Plant Breeding. The Macmillan Company.

Campbell, Lectures on the Evolution of Plants. The Macmillan Company.

Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Company.

Darwin, Different Forms of Flowers on Plants of the Same Species, D. Appleton and Company.

Darwin, Fertilization in the Vegetable Kingdom, Chaps. I and II. D. Appleton and Company.

Darwin, Orchids Fertilized by Insects, D. Appleton and Company.

Lubbock, British Wild Flowers. The Macmillan Company.

Müller, The Fertilization of Flowers. The Macmillan Company.