THE VEGETATIVE SYSTEM 681 
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. Trencilla) or Selaginella (S. pumila 
