26 The Nature and Origin of Stipules. 



capitulation to the development of plants we may arrive at valu- 

 able and trustworthy conclusions. The question would at once 

 be asked, where shall the embryology of the flowering plants be 

 studied, and the answer would naturally be, in the development 

 of the seed in the ovary. And here indeed, we trace in outline 

 an epitome of the course of development from the simple unicel- 

 lular organism, represented by the fertilized egg-cell of the ovule 

 to the highest thalloid form, the " embryo," with its bud (plu- 

 mule) which is to develop into the full-formed plant perfect in all 

 its parts. For a summary of the further development of the 

 Angiosperms we must look to the growing bud which is the 

 essential reproductive organ of the sporophyte stage and, doubt- 

 less, a more primitive one than the seed, for it is common among 

 the more ancient Pteridophytes and these have no seed. The 

 embryo of flowering plants does, however, correspond pretty 

 closely to that advanced stage of development of the egg-cell of 

 some of the higher Pteridophyta now generally spoken of as the 

 embryo and should be regarded as a j^oung plant in a state of 

 arrested development. In this state it remains during a period 

 of rest, in a highly specialized environment in the seed, await- 

 ing favorable conditions for farther growth. Because of the 

 highly specialized environment of the embryo, it has itself be- 

 come correspondingly specialized and has been variouslj' modi- 

 fied to suit the special conditions of its surroundings. The plu- 

 mule cannot then be regarded as any longer representing a prim- 

 itive form of bud and its development is so altered by secondary 

 modifications that the series of phylogenetic changes is disguised 

 and imperfectly represented. A parallel case is found among 

 animals in the development of Echinoderms, in which the changes 

 that have taken place through secondary modification are so great 

 that the relationship of the group cannot be satisfactorily^ deter- 

 mined by developmental evidence. 



It is not then in the seedling that we should expect to find rep- 

 resentations of primitive leaf-forms, though later ancestral forms 

 paralleling those of fossil leaves, of which we shall speak, are found 

 in some seedlings, as for example in Liriodendron. But it is in 

 the growth of the less specialized buds developing under more 

 primitive conditions that we should expect to find them. Such 

 buds are the ordinary leaf-buds of perennial plants, and especiall}^ 

 those occurring on basal and subterranean portions which I con- 



