HOMOLOGY OF ORGANS 63 



quite different in shape from other cotyledons 

 certainly, but like them, the first leaf organ of the 

 embryo. 



The cotyledon, we learnt at the beginning of this 

 chapter, is the first leaf of the young plant, and is 

 already formed in the seed. No one would call the 

 scutellum a leaf, but then neither would one at first 

 sight call the two cotyledons of the Broad-bean and 

 the Pea leaves. We argue that they are really leaves, 

 though changed out of all recognition on account of 

 the very different work that they have to perform, 

 because they are situated exactly as are those of the 

 Brown-bean which come out of the seed, and these 

 again exactly as those of the Melon or Castor which 

 are obviously of a leaf nature. 



10. We learn from this that the same kind of 

 organ, e.g. a leaf, may be quite different in shape and 

 character in different plants and in different parts 

 of the same plant, because of the different functions 

 it may have to fulfil. So that the shape and struc- 

 ture of an organ depends very largely if not altogether 

 On its functions ; and it is only by comparison with 

 similarly situated organs in other plants that we can 

 tell what it really corresponds to. If it were not 

 for the steps connecting the scutellum of the Maize 

 with ordinary cotyledons and leaves, we might well 

 conclude that it was a special organ produced, be- 

 cause it was wanted to obtain nourishment. 

 i We have had one instance of such a specially 

 produced organ, in the curious swelling which arises 

 on the hypocotyl of the seedling Melon and keeps 

 the seed-coat open. But there is nothing connecting 



