MORPHOLOGY 17 



monosymmetrical, dorsiventral foliage leaf is diagrammatically repre- 

 sented. From the surface view (A) and from the cross-section (B), 

 in which the distinction between the dorsal and ventral sides is in- 

 dicated by shading, it is obvious that but one plane of symmetry 

 (s) can be drawn. As the zoologists often term this degree of 

 symmetry BILATERAL, the same term is frequently employed with 

 reference to plants. 



Branch Systems 



Thallophytes as well as Cormophytes exhibit systems of branching, 

 resulting either from the formation of new growing points by the 

 bifurcation of a previously existing growing point, or from the develop- 

 ment of new growing points in addition to those already present. In 

 this way there are produced two systems of branching, the DICHOTO- 

 MOUS and the MONOPODIAL. By the uniform development of a continu- 

 ously bifurcating stem, a typical dichotomous system of branching is 

 produced, such as is shown in Didyota dichotoma (Fig. 8). In a 

 typically developed example of the monopodial system there may 

 always be distinguished a main axis, the monopodium, which gives 

 rise to lateral branches from which, in turn, other lateral branches 

 are developed. A good example of this form of branching is 

 afforded by a Fir-tree. Where one of the two branches is regularly 

 developed at the expense of the other, the dichotomous system 

 assumes an appearance quite different from its typical form. The 

 more vigorous branches may then, apparently, form a main axis, 

 from which the weaker branches seem to spring, just as if they 

 were lateral branches. This mode of branching is illustrated by 

 the Selaginellae (Fig. 351). Such an apparent main axis is termed, in 

 accordance with its origin, a Sym- 

 PODIUM. On the other hand, in the 

 monopodial system two or even 

 several lateral branches may develop 

 more strongly than the main axis, and 

 so simulate true dichotomy or poly- 

 TOMY. Such monopodial forms of 

 branching are referred to as FALSE 

 dichotomy or false polytomy, as 

 the case may be. A good example 

 of false dichotomy may be seen in the 



_,... . t-rr- i7 tt i/»\ Fig. 16.— Shoot of Viscum album, snowing 



Mistletoe (VlSCUm album, Dig. lb). false dichotomy. (J nat. size.) 



If, however, a lateral branch so ex- 

 ceeds the main axis in development that it seems ultimately to become 

 a prolongation of the axis itself, a sympodium is again formed. This 

 is exactly what occurs in the Lime and Beech; in both of these 

 trees the terminal buds of each year's growth die, and the prolonga- 



C 



