THE VEGETATIVE SYSTEM 683 
region below the branching: the latter commonly start afresh from simple 
beginnings, analogous to those of the seedling, with a contracted stele, and 
leaves of smaller size, and simpler form and arrangement. These facts seem 
to mark a distinction between terminal and accessory ramification. 
By either, or by both of these modes of branching, there is ample 
provision for extension of the shoot-system, over and above its own apical 
growth. The branchings, whether terminal, axillary, or adventitious result 
in the repetition of the original unit, modified, it may be, in certain minor 
respects, but retaining the essential characters of the primary shoot. But 
the upright position so common for the latter is not habitually maintained by 
the later derivatives, which show a tendency to run off into plagiotropic and 
dorsiventral modifications: not uncommonly they may take an underground 
course. And thus, primarily from its own apical growth, but secondarily 
from repetition of the primitive unit as a result of branching, the diverse 
vegetative systems of vascular plants are built up. 
There are certain analogies between the branching of the axis and 
that which is seen in the leaf of many vascular plants. In not a few 
cases the leaf is unbranched, and this—as in the case of the unbranched 
axis—may be held as a primitive condition, though very many cases where 
simple leaves exist have probably been derived by reduction from more 
complex types with branched leaves. But just as the axis may dichotomise 
in primitive forms, so also is dichotomy seen to be widely existent in the 
leaves of early vascular types, and examples come from all the phyla 
excepting the Lycopodiales. In the Equisetales, the ancient Asterocalamites 
had leaves repeatedly dichotomous (Fig. 199); and a somewhat similar 
branching of the large leaves existed in Pseudobornia: these show that 
though many of the fossil Kquisetales, and all the living ones have simple 
leaves, the capacity for their dichotomy existed in the race. In the 
Sphenophyllales the dichotomy of the leaf is an outstanding feature, and it 
is represented in the modern Psilotaceae: in the latter Zmesipieris is 
specially interesting, since, though normally the sporophylls dichotomise 
but once, repeated dichotomies occur occasionally in the middle of the 
fertile region; this suggests that the leaves possess capacities for branching, 
normally unrealised, but brought into existence where the nutrition is most 
effective. In the Ophioglossales branching of the leaf is also seen; some- 
times it is clearly dichotomous (Ophioglossum palmatum), but in Botrychium 
and Helminthostachys it is modified in the direction of a monopodial 
branching. It is, however, in the Filicales that branching of the leaf 
attains its climax; and the prevalent dichotomy, and transition to a 
monopodial branching show interesting analogies to what is seen in the 
shoot itself. 
The roots, which have been recognised as adventitious and accessory 
parts upon the shoot, also show a branching similar to that of axis and of 
leaf. In the Lycopods the roots are sometimes unbranched, as is usual 
1See p. 627, etc., where the literature is quoted. 
