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 flower some of the 

 structures become less and less like "leaves," 

 and more and more narrovA", until they are 

 definitely stamens, made up of the sfalklike 

 filament and the distinct anther 



