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 Equisetales, 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 Tmesipteris 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. 1 



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 

 1 See p. 627, etc., where the literature is quoted. 



