Cross-pollination of a Head (Clover). In a flower cluster called a head, a closely massed cluster of little flowers as clover, cross-pollination is usually effected by bumblebees which rapidly work from one flower to another in the same cluster, inserting their tongues deep into the flower cup. The butterfly shown in the illustration inserts its proboscis (seen curled up like a watch spring on the underside of the head) into the flower.

A composite head.

Cross-pollination of a Composite Head. This flower cluster, so often mistaken for a single flower, is found only in the great Composite family, to which so many of our commonest flowers and weeds belong. The daisy, aster, goldenrod, and sunflower are examples of the Compositæ.

The composite head is well seen in a daisy or the sunflower. This head has an outer circle of green parts. These parts look like sepals, but in reality are a whorl of leaflike parts. Taken together these

form an involucre. Inside the involucre is a whorl of brightly colored, irregular flowers called the ray flowers. They appear to act, in some instances at least, as an attraction to insects by showing a definite color (see the common dogwood, Cornus florida). The flowers occupying the center of the cluster are the disk flowers. Such a flower examined under the hand lens is found to be perfect. A careful observer will find that the anthers are united in a ring around the pistil. This is a typical condition in the Compositæ. The stamens ripen first and grow up around the stigma, which ripens later. The stigma splits (see a), and pollen from another flower brought to its surface will germinate there.

Section through composite head, showing a disk flower (a), a ray flower (c), and the involucre (d).

Other examples. 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 stamens caught in little pockets of the corolla. The weight of the visiting insect on the corolla releases the anther of the stamen from the pocket in which it rests, and the body of the visitor is dusted with pollen.

The milkweed or butterfly weed (Asclepias cornuti) is another example of a flower adapted to insect pollination.'

called the pronuba, gathers pollen from an anther, flies away with this load to another flower, there 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 been fertilized by
the pollen placed on the stigma
by the mother. They eat
some of the developing seeds
and then bore out of the seed
pod and escape to the ground,
leaving the plant to develop
the remaining seeds without
further molestation.

The fig insect (Blastophaga
grossorum) is another member
of the insect tribe that is of
considerable economic impor-

polli

Pronuba
nating pistil of
yucca.

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. For simple reference reading, see Coulter, Ilant Relations. A classic easily read is Darwin, On the Fertilization of Orchids.

tance. 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.1 The last two cases are only some of the many examples of mutual help among plants and animals.

Pollination by the Wind. Not all flowers are dependent upon insects for cross-pollination. Many of the earliest of spring flowers appear almost before the insects do. These flowers, needing no conspicuous colors or showy corolla to attract insects, often lack

this part altogether.

In fact, we are apt entirely to overlook the flowers which appear in the spring upon our common forest and shade trees. In many trees the flowers appear before the leaves come out. Such flowers are dependent upon the wind to carry pollen from the stamens of one flower to the pistil of another. Most of our

common trees, oak, poplar, maple,

and

others, are cross-pol

linated almost entirely

by the wind.

Among the adapta

tions that a wind-pollinated flower shows are: (1) The development of very many pollen grains to each ovule. In one of the insect-pollinated flowers, that of the night-blooming cereus, the ratio of pollen grains to ovules is about eight to one. In flowers which are to be pollinated by the wind, a large number

1 The teacher is referred to Yearbook of the Department of Agriculture for 1900 for data on the insect which pollinates the Smyrna fig.

of the pollen grains never reach their destination and are wasted. Therefore in such plants several thousands, perhaps hundreds of thousands, of pollen grains will be developed to every ovule produced. Such are the pines. In May and early June the ground under pine trees is often yellow with pollen, and the air may be filled with the dust for miles from the trees. Such, also, is the case with many of the grasses.

(2) The anthers are usually exposed to the wind when ripe. The common plantain and timothy grass are excellent examples. (3) The pistil of the flower is peculiarly fitted to retain the pollen by having feathery projections along the sides which increase the stigmatic surface. This can be seen in the grass. In the Indian corn the stigmatic surface is the so-called silk which protrudes beyond the covering of modified leaves which form the husk of the ear of corn. All our grains, wheat, rye, oats, and others, have the typical feathery pistil of the wild grasses from which they descended.

(4) The corolla is often entirely lacking. It would only be in the way in flowers that are dependent upon the wind to carry pollen. Imperfect Flowers. Some flowers, the wind-pollinated ones in particular, are imperfect; that is, they lack either stamens or pistils. In such flowers, cross-pollination must of necessity follow.

If only the staminate flowers (those which contain only stamens) are developed on one plant, and only the pistillate (those which bear only pistils) on another, we call the plant diacious. A common example is the willow.

Other plants bear staminate and pistillate flowers on the same plant. In this case they are said to be monacious. The oak, hickory, beech, birch, walnut, and chestnut are familiar examples.

The pine tree is another example of monoecious tree; the male or staminate flowers appear in tiny clusters called catkins, the female or pistillate flowers coming a little later as tiny cones, which in most species of pines take nearly two years to produce seeds.

Water Pollination. An unusual method of pollination is found in those plants which live almost entirely under the water. In eelgrass the pistillate flowers are attached to long, slender stalks and float on the

surface of the water. The staminate flowers, when ripe, break away from their submerged stems and float to the surface. If these float under a pistillate flower, the protruding ends of the pistils catch and retain some of the pollen from the staminate flower. Thus fertilization follows. After pollination, the stalk of the pistillate flower coils up in a spiral and draws the flower under the surface of the water, so that the seeds may ripen in security.

Summary.

If we now collect our observations upon flowers with a view to making a summary of the different devices flowers

Flowers of the Lady Washington geranium showing the conditions of dichogamy; A, flower with stamens ripe, but with the stigma not ready to receive pollen; B, the same flower at a later stage; the stamens have withered, but the stigma is now ready to receive pollen.

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. This condition is called dichogamy.

(3) The stamens and pistils may be so placed with reference to each other that pollination can be brought about only by outside assistance.

In some flowers, as is shown by the primula of our hothouses, the stamens and pistils are each of two different lengths in different