6.] 



CHAPTER I. GENERAL MORPHOLOGY. 



33 



Two principal types of branching may be distinguished (see 

 p. 19), the dichotomous and the lateral; but it must be borne in 

 mind that this distinction is somewhat arbitrary, and that these 

 types are connected by intermediate forms. 



1. In dichotomous branching, the apical growing-point divides 

 into two new growing-points (Fig. 8) which are at first equally 

 vigorous ; and they may (a) either remain so, or (6) the one may 

 develope more vigorously than the other. 



(a,) When the former is the case, the dichotomy developes in 

 a bifurcate manner (Fig. 18 A). The subsequent dichotomies 

 may all take place in the same 

 plane, as in the thallus of many 

 Liverworts, and in the leaves 

 of Schizcea dichotoma (Fern), so 

 that the whole branch- system 

 lies in one plane, and is bi- 

 lateral. Or the subsequent 

 dichotomies may take place in 

 various planes, which com- 

 monly intersect at right an- 

 gles, as in the roots of Isoetes, 

 where the branch-system is 

 radial. 



(6) When the latter is the 

 case, the branch-system be- 

 comes sympodial. The basal 

 portions, or podia, of the suc- 

 cessive bifurcations constitute 

 an axis, termed a pseud-axis, 

 or sympodium, on which the 

 weaker branches appear as 

 lateral out-growths (Fig. 18 

 B, (7). The more vigorous 

 branches may be always those 

 of one side, producing what is termed a helicoid or hostrychoid 

 dichotomous branch-system (Fig. 18, B}, as in the thalloid shoot 

 of Fucus ; or they may be developed alternately on opposite sides, 

 when a scorpioid or cincinnal dichotomous branch-system is pro- 

 duced (Fig. 18, 0). 



Dichotomous branching is the less common type. It occurs in some Algae, 

 (e.g. Cladostephus, Dictyota dichotoma. Fucus, some Floridese) ; in some Liver- 

 V. S. B. D 



FIG 18. Diagram of the various modes of 

 development of a Dichotomy. A Bifurcate 

 dichotomy. B Helicoid dichotomy; here 

 the left-hand (I) branch is always more 

 vigorous than the right (r). C Scorpioid 

 dichotomy ; the right and left branches are 

 alternately more vigorous in their growth. 



