PART I. MORPHOLOGY. 



[12 



axis of the former is perpendicular to the plane of expansion of 

 the latter. At first the development is that of a palmate leaf, 

 the petiole being inserted at the base of the lamina, and at the 

 point of insertion there is an intercalary growing-point from which 

 spring several axes (Fig. 22 B, i 2,3) in basipetal succession. But 

 in this case the last-formed axes(j-,5-5, in the. figure) grow out 

 in front of the petiole, with the result that the whole lamina 

 gradually comes to lie perpendicularly to the petiole. 



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 pri- 

 mary axis (epipodium) of a 

 pinnate lamina is termed a 

 mid-rib. 



The Branching of the Leaf 

 is commonly confined to the 

 epipodium, and then it takes 

 place in the lateral plane ; 

 less commonly it occurs in 

 the mesopodium (e.g. species 

 of Ophioglossum, Botry- 

 chium, Marsilea), and then 

 (as in these plants) the 

 branching frequently takes 

 place in the antero-posterior 

 (or dorsiventral) plane. 



The branching of the epi- 

 podium is, like that of a stem 

 or a root, either dichotomous 

 or lateral, and essentially the 

 same forms of branch-systems 

 are produced. Dichotomous 

 branching is comparatively 

 rare ; it has been observed in the Hymenophyllacese, the branches 

 either remaining distinct or forming sympodia. The two first 

 leaflets of Marsilea are said to be formed by dichotomy. 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. 21 C ; of Helleborus, and of some Aroids) there is 

 apparently cymose branching with formation of a sympodium. 

 The ribs of the lamina are branches of the epipodium. The 



Fio. 22. Development of peltate leaf of Hy- 

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

 young; C somewhat older (B and 0x60); S 

 petiole ; 1-5 primary axes of growth in young 

 leaves, ribs in adult leaf; a secondary axes of 

 growth. 



