Unity of Life 95 



** leaves " masquerade as spathes, as involucre scales or bracts, 

 as sepals, as petals, as stamens, as carpels. They thus fur- 

 nish the burr of a nut or the flesh of an apple and the pod of 

 a bean. The pitcher of the pitcher plant, the trap of the 

 Venus's flytrap or of the sundew, a layer of onion, all these are 

 ** leaves." The flower of the 

 common water lily shows the 

 gradation from sepal, through 

 petal and stamen to carpel 

 (Fig. 26). 



Unity of Life 



Fig. 26. Homology in the 

 Structure of a Flower 



As we pass toward the center of the 

 flower of the water lily the structures 

 of each circle become less and less like 

 " leaves," and more and more narrow, 

 until they are definitely stamens, made 

 up of the stalklike filament and the 

 distinct anther. From Gruenberg, 

 Biology and Human Life, published 

 by Ginn & Company. 



The great variety of ap- 

 pearance and function assumed 

 by fundamentally identical 

 structures may be explained by 

 considering them adaptations 

 to different modes and condi- 

 tions of life. We may say, if 

 we like, that the limb of a 

 whale differs from that of a camel or of a bird because it 

 has to serve its owner in a different way. The lobster with 

 a whole series of " legs " uses the ones on the head for cer- 

 tain purposes and those on the abdomen for other purposes. 

 There still remains, however, the question of remoter relation- 

 ship. How does it come about that different classes of back- 

 boned animals have the same fundamental structures to begin 

 with? We can understand that a plant can put forth leaves 

 and have a great variety of ** leaves " serving a variety of 

 functions. They are all parts of the same organism and 

 contribute to the life of the whole; there is a division of labor 

 among structures that are specialized as the plant develops. 

 In a similar manner we might understand that the lobster 

 and other arthropods can produce outgrowths or append- 

 ages which become differentiated or specialized in the course 

 of development. It is not so easy to understand the funda- 



