Volume 3 - No. 3
LESSONS IN BOTANY
By Dr. W. B. McDougall,
The beauty of the surface of the earth depends very largely upon the plants that grow upon it. One can scarcely imagine what a drab and uninteresting place this world would be, without the trees, the shrubs, the grass, and the flowers. When we go on a hike or an automobile ride, the amount of pleasure that we derive from the outing is measured to a large extent by the character and luxuriance of the plant life that we encounter, and especially upon the numbers of wild flowers that we see. Since we derive so much enjoyment from plants, we like to have a speaking acquaintance with at least some of those that we come upon. There are so many different kinds that no one person can know all of them well enough to call them by name, but it is a simple matter to become acquainted with a few of the common ones in our immediate vicinity, and to increase our knowledge, on each of our outings.
The fundamental parts of a seed plant are roots, stems and leaves. Sometimes these parts may be so modified that they do not look much like roots, stems or leaves. In order that a seed plant may perpetuate itself through the production of seeds, it must produce flowers. But a flower is merely a modified stem bearing modified leaves. In a typical flower there are four sorts of modified leaves attached to the modified stem. The outermost set is made up of modified leaves called sepals. They are usually green, and look very much like little leaves. Collectively they make up what is called the calyx of the flower. Within the calyx is another set of modified leaves called the petals. These look like little leaves but they usually are not green. They may be white, red, yellow or blue, or almost any other color except green. The pet als, collectively, make up the corolla of the flower.
In the third set of modified leaves, within the corolla, are the stamens. They do not look like leaves at all. Usually a stamen consists of a stalk, called the filament, on the upper end of which is a sac-like or box-like structure called the anther. Within the anther are the little pollen grains which contain the male elements of the flower. Finally, in the center of the flower, we find one or more modified leaves called pistils. These, like the stamens, do not look like leaves. A pistil usually consists of three parts: an enlarged lower portion called the ovary, within which are found the ovules that contain the female elements of the flower, and that later may develop into seeds; a slender portion called the style; and a slightly enlarged upper end celled the stigma.
Sepals, petals, stamens, and pistils, then, are the four kinds of floral parts that we need to know about in order to identify different kinds of flowers. Suppose, for example, that we were to look at a buttercup flower. It usually has five green sepals, five or more yellow petals, many stamens, and several pistils. Suppose we remove one of the sepals so as to leave only four; remove one of the petals so as to leave only four; remove all of the stamens except six, and shorten two of the six; and remove all of the pistils except one. We would then have left the correct materials for making a mustard flower. Again, suppose we were to take the same buttercup flower and leave the five sepals just as they are; leave five petals but fasten them together to make a tube-shaped corolla; take off all of the stamens except five and fasten these five to the corolla; and remove all of the pistils except one. We would then have a primrose flower.
Thus we might continue, making all of the many different kinds of flowers without adding anything new, but by simply changing the number, size, shape, color, and arrangement of the four kinds of organs that we found in the buttercup. That is exactly what Nature has done. And that is why it is so important that we understand these four kinds of organs: the sepals, petals, stamens, and pistils.
Before a plant can produce seeds it is necessary that pollen grains be transferred from the anthers of the stamens to the stigmas of the pistils. This transfer is called pollination, and is a necessary prelude to fertilization which enables the ovules to develop into seeds. In the great majority of plants pollination is brought about through the agency of wind or insects. The wind will blow, whether flowers are attractive or not; but insects must be attracted if they are to perform their function. Therefore, beautiful flowers are simply devices to attract insects.
With the above brief introduction as a basis for beginning our study of flowers, suppose we start by learning something of the Yucca, often called Spanish Bayonet, because of the shape of the leaves. The Yucca is the state flower of New Mexico, but it is by no means limited to that state. When this story is published, in July, there will be one or more kinds of Yuccas blooming somewhere in more than half of the United States. These various kinds of Yuccas vary much in their general appearance and habit of growth, from the Spanish Bayonet of the east which has such a short stem that the cluster of leaves is right at the ground; to the Giant Yucca of southwestern Texas, and the Joshua Tree of Arizona and California. Some of the latter are over 25 feet tall. Plants are classified, however, primarily on the basis of their flowers and fruits, and we find that the flowers of the various kinds of Yucca are all very similar.
The Yucca belong to the lily family. The flowers are white, fairly large, and pendulous. They hang from the stem, upside down like a bell. The three sepals are nearly as white as the three petals. In fact, the sepals and petals look very much alike. There are six stamens and one large pistil. The stigma is bowl-shaped and, since the pistil is a little longer than the stamens, it is impossible for pollen grains to fall from the anthers onto the stigma. The Yucca is pollinated exclusively by a little white, woolly moth. If you examine the flowers of a Yucca plant during the day, you may find one or more of these little moths resting in a flower with their heads upward toward the base of the flower.
If you wish to see these interesting insects work, go back to the plants in the evening just after sundown, but before it begins to get dark. Locate a moth and keep watch of her. In a little while she will probably turn around with her head in the opposite direction. Presently she will fly to another flower, either on the same plant or on another plant. You will have to keep close watch so as not to lose sight of her. She will alight upon a stamen and collect pollen from the anther, placing the pollen in a little pouch on the under side of her head. Then she will fly to another flower, but this time she will alight upon the pistil. She will pierce the ovary with her ovipositor, lay an egg, and then creep down to the end of the pistil, take a bit of pollen from the pouch where she has it stored, and stuff it into the bowl-shaped stigma, butting it with her head to make it stick.
Probably the moth will repeat this performance of laying an egg and then placing pollen on the stigma in several different flowers before it becomes too dark to watch her. The moth, of course, performs these duties instinctively. It is difficult to imagine how evolution could bring about so strange an instinct, causing a moth to place pollen on the stigma of a flower every time she lays an egg, but the relationship is obligate for both the plant and the insect; that is, the moth cannot complete its life cycle without the Yucca plant, and the Yucca cannot produce seeds without the help of the moth. When there is a hatching of eggs that have been laid in the ovary, the larvae feed upon the ovules, of which there are usually many. The ovules that are not eaten develop into seeds.
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