THE VEGETATIVE SYSTEM 68 1 



seems unavoidable that these minor organs arose phyletically by enation, 

 as new outgrowths, from a previously smooth surface. If it be admitted 

 for emergences and hairs that new organs, not pre-existent in the race, can 

 originate by enation, are we to take a different view for leaves, notwith- 

 standing that the facts of individual development by enation are alike in 

 both cases? Is the leaf to stand alone among the appendages of the 

 shoot in having been fashioned from some pre-existing organ? It may 

 well be asked whether this view has any other foundation than in pre- 

 conception apart from fact. The ontogeny is against it. The phylogeny 

 does not show it to be a necessary view. Comparison with other 

 appendages of the shoot gives it no support. And, finally, its acceptance 

 has led its adherents into theoretical difficulties involving hypothetical 

 organisms such as " Archegoniate Algae " ; or a " Prohepatic " type has been 

 assumed. These appear as unnecessary as they are non-existent to those 

 who accept the guidance which the individual development gives with so 

 great constancy. It may, on the other hand, be urged that leaves are 

 essentially different from emergences and hairs : that they are more constant 

 in occurrence, and more regular in position, as well as physiologically 

 more important, as they were also prior in descent. But such differences 

 do not indicate a radical difference in their mode of origin : the early 

 phyletic appearance and physiological importance of the leaf would rather 

 lead one to expect that just such priority and regularity should rule in 

 their organisation as distinguishes them from the other appendages of the 

 shoot. On these grounds it is held that the phyletic origin of the leaf by 

 enation, like that of emergences and hairs, is more probable than any theory 

 under which it would be fashioned from some pre-existing organ, hitherto 

 undefined, and wholly hypothetical. 



The other appendages the roots bear no direct relation to the 

 continued apical growth of the axis. This fact, together with the great 

 diversity of their position and time of origin indicates them as accessory 

 parts as they have already been held to be in the primary embryogeny. 

 Thus whether from the primary embryogeny, or from the plant showing 

 continued apical growth, the conception of the simple shoot emerges ; it 

 is composed of a pre-existent axis defined in relation to the first 

 cleavage of the zygote ; upon this axis leaves are produced laterally, by 

 enation in acropetal order, also, though less constantly, emergences and 

 hairs; while the roots, and even the first root of the embryo, are 

 accessory organs. 



The simple shoot thus constituted, forms the unit upon which the 

 vegetative region of all Vascular Plants is built. Comparison indicates 

 that the radial construction of the shoot was primitive for the sporophyte, 

 and that where dorsiventrality occurs, it has been secondarily acquired 

 (Chapter XVI.). Such a shoot, developed as it is directly in the embryogeny, 

 may sometimes remain entirely unbranched : this is seen in some of the 

 simplest species of Lycopodium (e.g. L. Trendlla) or Selaginella (S. pumila 



