. Biology and human life. Biology. 52 BIOLOGY AND HUMAN LIFE. Fiff. 2' Homology in the structure of a flower and 5, Fig. 197). All the various structures that we may con- sider as having the same kind of origin (in this case, outgrowths from the stem) and the same fundamental structure (in this case, a more or less flattened structure with nerves running through it) are said to be homologous (see section 28). Nearly every beginner in the study of flowers is deceived by the daisy and the dandelion, of the sunflower family, because these plants have structures that are analogous (see section 29)
. Biology and human life. Biology. 52 BIOLOGY AND HUMAN LIFE. Fiff. 2' Homology in the structure of a flower and 5, Fig. 197). All the various structures that we may con- sider as having the same kind of origin (in this case, outgrowths from the stem) and the same fundamental structure (in this case, a more or less flattened structure with nerves running through it) are said to be homologous (see section 28). Nearly every beginner in the study of flowers is deceived by the daisy and the dandelion, of the sunflower family, because these plants have structures that are analogous (see section 29) to those we have already studied, but not homolo- gous. In this family of plants the flowers are very small, but many of them are clus- tered in a head, so that we commonly speak of the whole head of a hundred or more flowers as a flower. Certainly a head has the general appearance of a flower, and we may consider it analogous to a flower since, like a larger corolla, it at- tracts insects, as in the daisy or the sunflower, where the small flowers around the outer edge of the head have elongated or strap-shaped corollas that are sometimes mistaken for single petals (see Fig. 184). At the base of the head are many small, leaflike structures which we may consider analogous to a calyx. This structure is called an involucre, and the single leaves are called bracts. In the Jack-in-the-pulpit and the calla lily (both of the Arum family, not lilies) many tiny flowers are arranged on a spike, and a very large bract, sometimes mistaken for a corolla, sur- rounds the whole (see Fig. 26). In the dogwood the four large white or pinkish ^'petals" are really bracts. Thus we see that in plants as well as in animals a structure carries on the func- In the water lily (as well as in the peony and some other flowers) it is possible to see that the stamen may be considered as a special kind of leaf. There is a gradual passing from sepal to petal, and as we pass toward the center of the flowe
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