DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM. 



l8l 



the axis of growth of each shoot and through that of its immediate mother-shoot; 

 then, in the helicoid cyme each following median plane always stands right or 

 left of the preceding one, following the course of the leaf-spiral ; in the scorpioid 

 cyme, on the other hand, the consecutive median planes stand alternately right and 

 left. 



(a) In Thallophytes and the Thalloid Hepaticae, dichotomy is very common, but 

 monopodial branchings also occur developed in the most various ways. The dichotomous 

 branching is unusually clear and generally bifurcate among Algae, especially in Dictyoteae 

 and species of Fucus (in particular F. serratus). In some there occurs a tendency towards 

 a sympodial development of the bifurcations, but usually only at a late period ; so 

 that the dichotomous branching can be clearly recognised at the ends of the branches 

 even with the naked eye. The same is the case, among Hepaticae, in Anthoceroteae, 

 Riccieae, Marchantieae, and in Met%geria (Fig, 137), where a flat expansion of the thallus 

 or thalloid stem arises between 



the young bifurcations, first of 

 all ' as a protuberance (/' y"), 

 which however cannot be con- 

 sidered as a continuation of 

 the shoot, since it has no apical 

 cell or mid-rib ; subsequently 

 this protuberance disappears, 

 asin/'"\ 



Distinctly monopodial (la- 

 teral) ramifications are parti- 

 cularly clear in filamentous 

 Algae, when the apical cell re- 

 mains unbranched, and lateral 

 branches grow only out of the 

 individual cells (segments of 

 the filament) ; as in Cladophora, 

 Lejolisia, &c. It occurs how- 

 ever sometimes that lateral 

 branches proceed out of the 

 apical cell itself, as is espe- 

 cially shown in Stypocaulon (Fig. 108, p. 139). In other cases the branching of the 

 apical cell is dichotomous, as in Coleocheete soluta (see Book 11, Algae). 



(b) In the roots of Ferns, Equisetaceae, and Rhizocarpeae, as well as in those of 

 Conifers, Monocotyledons, and Dicotyledons, the branching is always, as far as is known, 

 at first monopodial, and even at a later period the primary root generally remains stronger 

 than its lateral roots ; these root-systems are therefore developed in a racemose manner 

 (Fig. 123, p. 165); this is seen very beautifully in the root-systems which proceed from 

 the primary roots of Dicotyledons when they are allowed to germinate and grow in water. 

 Dichotomy of roots occurs only in Lycopodiaceae, and probably in Gycadeae, where they 

 appear at a later period as systems of bifurcations. According to the most recent 

 researches of Nageli and Leitgeb, it is still altogether doubtful whether the branching 

 depends, even in Lycopodiaceae, on true dichotomy'^; but the root-branches of Lycopo- 



FlG. 137.— Flat thallus of Metzfreria fnrcata branching dichotoniously 

 (X about 15) ; m vi mid-rib consisting of several layers, branching dichoto- 

 mously at ^; ss apical points of the branches; yythe wing-like expansions 

 of the thallus, consisting of one layer of cells ; /' f" the wings between the 

 mid-ribs of younger branches. (The left-hand figure is seen from below, the 

 right-hand one from above.) 



^ For the above-mentioned reasons I share Kny's view that the branching is in this case 

 dichotomous. (See Hofmeister, Allgemeine Morphologic, p. 433.) 



^ Compare Nageli's Beitrage ziir wissen. Botanik, Heft IV, 1867. I would lay less stress on the 

 relation of dichotomies to the apical cell, because the latter has scarcely the same decided signifi- 

 cation in Lycopodiaceae as in Ferns, Equisetacese, and other Cryptogams ; and the apical growth 

 apparently approaches nearer to that of Phanerogams. 



