12.] CHAPTER II. SPECIAL MORPHOLOGY OF THE MEMBERS. 51 



v a, 



I 



The main axes of growth frequently grow thicker than the 

 rest of the lamina, so that they project as ribs on the under 

 surface. The thickened primary axis (epipodiuni) of a pinnate 

 lamina is termed a mid-rib. 



The Branching of the Leaf is commonly confined to the epipo- 

 dium, and then it takes place in the lateral plane ; less commonly 

 it occurs in the mesopodium (e.g. species of Ophioglossum, Bo- 

 trychium, Marsilia), and then (as in these plants) the branching 

 frequently takes place in the antero-posterior (or dorsiventral) 

 plane. As a rule the primordial leaf is unsegmented, though in the 

 foliose Jungermanniese it consists of two segments from the first. 



The branching of the epipodiuni is, like that of a stem or 

 a root, either dichotomous 

 or lateral, and essentially 

 the same forms of branch- 

 systems are produced. Di- 

 chotomous branching is com- 

 paratively rare : it has been 

 observed in the Hymeno- 

 phyllaceoB, the branches 

 either remaining distinct or 

 forming sympodia. The two 

 first leaflets of Marsilia are 

 said to be formed by dicho- 

 tomy. Lateral branching is 

 the more common form, and 

 the resulting branch -systems 

 are typically monopodial. 

 But in some cases (e.g. leaf 

 of Plane, Fig. 30 G ; of Helle- 

 borus, and of some Aroids) 

 there is apparently cymose 

 branching with formation of 

 a sympodium. 



The ribs of the lamina represent distinct axes of growth ; they 

 are, in fact, branches of the epipodiuni. The degree of seg- 

 mentation of the lamina depends upon the relation between the 

 growth of the various main axes and the marginal growth of 

 their respective wings (see Figs. 30 and 31). When these keep 

 pace with each other the lamina is altogether unsegmented, 

 that is, its margin is entire ; when the growth of the axes is 



FIG. 31. Development of peltate leaf of 

 Hydrocotyle : A full-grown (nat. size) ; B very 

 young; C somewhat older (B and 0x50); 

 S petiole ; 1-5 primary axes of growth in young 

 leaves, ribs in adult leaf; a secondary axes of 

 growth. 



