i6o 



EXTERNAL CONFORMATION OF PLANTS. 



the system arises alternately right and left, as in Fig. 128, A, B, the system is 

 a Scorpioid Cyme (or cicinus). If in these cases we have to do with leaf-forming 

 shoots with a spiral arrangement of the leaves, a more exact definition of the 

 terms right and left becomes needful. It is then necessary to imagine a median 

 plane drawn through the longitudinal axis of each shoot and through that of its 

 immediate mother-shoot; then in the helicoid cyme each following median plane 

 always stands either 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 thalloid Hepaticee, dichotomy is very widely prevalent; 

 monopodial branchings also occur in the most various degrees of development. The 



dichotomies are unusually clear 

 and generally developed in a 

 forked manner among Algae, 

 especially in the Dictyotese, and 

 species of Fucus (in particular F. 

 serratus). In some there occurs 

 a tendency towards sympodial 

 development of dichotomies, but 

 usually only at a late period ; 

 so that in those which branch 

 the dichotomy can be clearly 

 recognised even with the naked 

 eye. The same is the case, 

 among Hepaticae, in the Antho- 

 cerotese, Riccia, Marchantia, and 

 Metzgeria (Fig. 129), where the 

 flat extension of the thallus or 

 thallus-like stem arises between 

 the young branches, first of all as 

 a protuberance (/" y"), which 

 however cannot be considered 

 as a continuation of the shoot, since it has no apical cell or mid-rib ; subsequently this 

 protuberance disappears, as mf"^. 



Decidedly monopodial (lateral) ramifications are particularly clear in filamentous 

 Algae, when the apical cell remains unbranched, and lateral branches grow only out of 

 the individual cells (segments of the filament) ; as in Cladophora, Lejolisia, <fec. It also 

 however sometimes occurs that lateral branches proceed out of the apical cell itself, as 

 is especially shown in Hypocaulon (Fig. 98, sect. 19). In other cases the branching of 

 the apical cells is dichotomous, as in Coleochoete soluta (see Book II. Algae). 



(b) In the roots of Ferns, Equisetacese, and Rhizocarpeae (according to Nageli and 

 Leitgeb), as well as in those of Conifers, Monocotyledons, and Dicotyledons, as far as is 

 known, the branching is always at first monopodial, and even when further developed 

 the mother-root generally remains stronger than its lateral roots ; these root-systems are 

 therefore developed in a racemose manner (Fig. 113, p. 143) ; this is seen very beauti- 

 fully in the root-systems which proceed from the roots of Dicotyledons when they are 

 allowed to germinate and to grow in water. Dichotomy of roots occurs only in Lyco- 

 podiacese, and probably in Cycadeae, where they appear in their further development as 

 systems of bifurcations. According to the most recent researches of Nageli and Leitgeb, 



Fig. 129. — Flat dichotomously branched thallus of Metzgeria ficrcaia 

 (X about 15); »t7jt mid-rib consisting ofseveral layers, dichotomously branched 

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

 thallus, consisting of one layer of cells ; f'f" the coherent 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. (Cf. Hofmeister, Allgemeine Morphologic, p. 433.) 



