CHAPTER XV 



EXAMPLES OF HOMOLOGY 



IN chapter ii we learnt that the three principal vege- 

 tative organs of a plant are its stem, its roots and 

 its leaves, and while studying the germination of 

 seeds, we saw that leaves are capable of considerable 

 modification, and may indeed be so altered in general 

 appearance, that their leaf nature is at first quite 

 unrecognizable. The modification in the form of the 

 embryo's leaf is connected, we saw, with the particular 

 work it had to perform, and the particular circum- 

 stances in which it was placed. The cotyledons for 

 instance, even when as with RICINUS, the Castor plant, 

 they come out of the seed and turning green behave 

 thereafter like true leaves, are simple leaves with 

 entire margin, however lobed or toothed the normal 

 leaves may be, and this, we saw, is because they thus 

 fit best into the seed, with least waste of room. We 

 learnt too, that some cotyledons are still more unlike 

 leaves, because they are used as stores of food for the 

 seedling plant, the culmination of such modification 

 being reached in the scutellum of the Maize, which 

 is utterly different in every way from the ordinary 



