i84 



MORPHOLOGY OF MEMBERS. 



dcvclopes further only on one side, the new branches always arising either only on the left 

 or only on the right side, 3 from 2, and 4 from 3 ; every lateral branch thus produces a 

 sympodial system, and in fact a helicoid cyme. 



If now the basal pieces 2, 3, 4, combined in a sympodial manner on both lateral shoots, 

 are imagined to be much shortened, so that the bases of the lobes 2, 3, 4 come close to 

 the base of the lamina i, then all the lobes of the leaf will appear to spring from one 

 point, and the leaf is called digitate. It would appear, however, that such leaves may also 

 arise by the formation from the broad end of the young leaf itself, first of a middle lobe, 

 and then of new lateral lobes right and left from above downwards, as in Lupinus, 

 according to Payer's drawings (Organogenie de la fleur, pi. 104). If the lobes remain 

 completely United or have the appearance of a continuous lamella, we have a peltate 

 leaf \ It is impossible to go more into the detail of these processes without numerous 

 illustrations which cannot be given here. Fig. 142 will explain, in conclusion, the 

 origin of the quadripartite lamina of the leaf of Marsilea Drummondi, according to 



Hanstein (Jahrb. fur. 

 wissen. Bot. vol. IV). 

 The leaf has its origin 

 in a cell of the cone 

 of growth of the stem, 

 which, becoming the 

 apical cell of the leaf, 

 produces two rows of 

 segments from which 

 the right and left halves 

 are formed. Thus a 

 broad cone first arises, 

 growing at its apex, and 

 bent towards the stem 

 {A, B) ; when this, 

 which is the future 

 petiole, has attained a 

 certain height, it in- 

 creases in breadth right 

 and left. Beneath the 

 still growing apex, Z), ^j, 

 a protuberance {stb) arises on both sides ; and while the latter (destitute of an apical 

 cell) becomes still more arched (C, stb), the apical growth of the leaf ceases (C, bs)^ its apical 

 cell disappears, and soon two equally strong outgrowths arise near the apical point, which, 

 like the earlier lateral ones, increase vigorously and grow out into broad lobes of the leaf. 

 Thus arises a quadripartite lamina at the end of the petiole, the lateral lobes of which 

 have resulted from lateral branching, but the middle ones by dichotomy. The four 

 lobes remain, as they grow, narrow at their base, becoming much broader at the free 

 margin ; and, since the part of the leaf from which they originated remains short and 

 narrow, they appear, in the mature leaf, to spring from a single point, the end of the 

 petiole. 



(d) Branch-system of Leaf-bearing Shoots. The branching of the stem of Lyco- 

 podiaceae is dichotomous. In Psilotum triquetrum all the branches develope uniformly; 

 and this is the most regularly developed dichotomy found among vascular plants. 

 In Lycopodieae the development is much more irregular, but the bifurcation is 

 always evident throughout; in Selaginelleae, on the other hand, it is generally to be 



Fig. 142. — Development of the leaf of Marsilea Drn^nmondi (after Hanstein). A, C, D 

 seen from the inner surface; B longitudinal section vertical to ^ ; bs apex of the leaf; 

 g—z the segments of the apical cell ; stb lateral lobes of the lamina in their earliest state. 



^ Compare further Trecnl, Formation des feuilles, in Ann. des Sci. Nat. vol. xx. 1853; and 

 Payer, I.e. p. 403; also Entwickelung der Blattgestalten, Jena 1846. 



