Volume 3 - No. 4
LESSONS IN BOTANY
By Dr. W. B. McDougall,
The largest of all the families of flowering plants is called the composite family. In the entire world there are probably more than 15,000 different species of composites and, as a general rule, about one-fifth of all the flowering plants in any given locality belong to this family. They grow in all conceivable sorts of habitats from sea level to the tops of high mountains, and from dry deserts to swamps and lake margins. Since they are so numerous and so widely distributed it is important that we know something of them.
Some of the well known plants belonging to the composite family are the asters, goldenrods, thistles, ragweeds, dandelions, sunflowers, daisies, lettuce and sagebrush. The reason they are called composites is that the so-called flower, of a dandelion or an aster, for example, is, in reality, not a single flower but a bouquet of flowers; that is, it is composed of numerous flowers all clustered on one flower stem.
Let us examine the flower of a dandelion. If we take a dandelion "head" which is in full bloom, and pull off one of the yellow petal-like structures, we will have a single dandelion flower. At the lower end of this flower we will find a whitish body which is nearly cylindrical but pointed at the lower end. This is the ovary which will later develop into a one-seeded fruit. At the upper end of the ovary there is a short stalk, and attached to the end of this stalk are numerous white hairs. These hairs are collectively called the pappus, and they represent the calyx of the flower. In some members of the family the pappus consists of bristles, awns, scales, or teeth, and in some cases it is lacking entirely.
The yellow, strap-shaped portion of the dandelion flower is the corolla, and the five teeth at its end indicate that it consists of five petals grown together. It will be noted that the lower end of this corolla is not flat but tubular, and through this tubular portion extends the style of the pistil with two stigmas at the upper end, each curled outward. The filaments of the five stamens are included within the tubular part of the corolla but the rather long anthers are united around the style just above this tubular part.
Thus we see that the little dandelion flower has all of the parts that we are familiar with in flowers, and that what we have commonly thought of as a dandelion flower consists of a large number of flowers all attached to the end of a stem which is much enlarged and expanded and is called a receptacle. Surrounding the cluster of flowers on the receptacle are numerous green bracts. These are modified leaves, and make up what is called the involucre.
All of the flowers in a dandelion "bouquet" are alike. If we examine a sunflower head, however, we find that the bouquet is made up of two kinds of flowers. Those around the margin of the bouquet have flat corollas and are similar to these of the dandelion, but those in the central portion of the bouquet have short, tubular corollas with no flat portion. These latter are called disk flowers, while the flowers with flat corollas are called ray flowers. In some sunflowers both the ray flowers and the disk flowers are yellow but in other the disk flowers are purple or brown and the ray flowers yellow so that there are two colors as well as two kinds of flowers in the little bouquet. In the asters this is almost always true, the disk flowers being yellow and the ray flowers usually either purple or white.
The advantage of having a large number of flowers clustered together on one receptacle is obvious. They can be small and still conspicuous enough to attract insects. Once an insect has been attracted it is likely to visit a number of flowers before leaving. In some kinds of composites the ray flowers do not produce any fruits, their function being merely to attract insect while the disk flowers produce the fruits.
In addition to the two groups of composites described above, that is, those that have both ray flowers and disk flowers and those that have only the ray type of flowers, there is a third group in which all of the flowers are disk flowers. This is true, for example, of the Joe Pye weed and the white snakeroot of the East, and of the rabbitbrush and the pincushion flowers of the West.
Although the composite family is the largest of all plant families, relatively few of its members are of value to man except for ornamental planting and cut flowers. Lettuce and, in some places, dandelion greens, are the only food plants from this family that are extensively used in the United States. On the other hand there is a large number of pernicious weeds in the family.
The great size of the composite family and the large numbers of weeds in it indicate that it has been a remarkably successful family. This is all the more striking becauses it is believed that this is one of the youngest of plant families. Why has the family been so successful? Probably the most important reason is to be found in the very efficient means that have been developed for scattering the seeds. The composites produce one-seeded fruits. A sunflower "seed" or a dandelion "seed", for example, is a one-seeded fruit and in all cases, therefore, the entire fruits rather than the seeds alone are scattered.
Some composites, such as cockleburs and beggar-ticks, produce fruits that readily cling to the hair of animals or the clothing of men and are thus widely scattered. The great majority of composites, however, depend upon wind as a fruit-scattering agent.
Referring again to the familiar dandelion we find a most remarkable adaptation for wind-scattering of fruits. If we examine a dandelion head while the flowers are still in bud we will find that the innermost bracts of the involucre are completely enclosed around the flower buds in a protective manner. When the flowers bloom these bracts bend at the middle, the tips being turned outward to permit the flowering head to expand. After the flowers have been pollinated by visiting insects they wither, and the bracts of the involucre close up again to protect the developing fruits.
While the fruits are developing, two important changes take place. First, the flower stalk, which was relatively short at blooming time begins to elongate, and the length to which it grows depends on the height of the surrounding vegetation. Such flower stalks have been known to become as much as 3 feet long when the dandelion plant was growing in a patch of tall weeds. In any case, this elongation of the stalk places the head in such a position that it is fully exposed to the wind. At the same time the little stalk at the tip of the ovary to which the pappus is attached elongates to such an extent that when the fruit is fully mature the pappus is at some distance from the fruit, and the fruit with the pappus forms a veritable parachute which may be carried for miles on a light breeze.
Finally when the fruits are ripe and ready to be scattered the involucre opens again and this time the bracts do not bend at the middle as they did when the flowers were in bloom. They bend at the base and become turned back entirely out of the way so that there is nothing to prevent the wind from picking up the little parachute-like fruits and scattering them far and wide.
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