12. THE LEAF. 



33 



tudinal axes of both the stem and the leaf, cuts the lamina into 

 two lateral halves ; in other words, it is so placed that its upper 

 (ventral) surface faces the apex of the stem, and its lower (dorsal) 

 surface is directed away from it. As a rule, the two lateral halves 

 of the lamina are symmetrical; but in some cases (e.g. Elm, 

 Begonia) they are unsymmetrical, when the lamina is said to be 

 oblique. 



The ultimate form of the lamina mainly depends upon the 

 degree of elongation of the epipodium. When the epipodium 

 elongates considerably, the lamina has a well-marked primary 

 axis from which more or less numerous secondary axes of growth 

 successively spring, and these in turn bear lateral axes of a higher 



Fie. 21. A Pinnate leaf of the Beech, Fagus syloatica; m mid-rib, n lateral ribs. B Pal- 

 mate leaf of Alchemilla vulgaris (nat. size.) C Pedate leaf of the Plane (} nat. size). 1, 2, 

 3, are the ribs or axes of the 1st, 2nd, and 3rd order. 



order : the resulting lamina is then of the pinnate type (Fig. 21 

 A). When, however, the epipodium remains short, it constitutes 

 merely an intercalary growing-point from which a number of 

 equal secondary axes spring, and the resulting lamina is of the 

 palmate type (Fig. 21 B). 



The development of the peltate lamina, closely connected with 

 that of the palmate type, is effected by a peculiar form of basipetal 

 growth. In peltate foliage-leaves (e.g. Tropseolum, Nelumbium, 

 Hydrocotyle, Cotyledon, Lupinus, etc.) the petiole is inserted in 

 the middle of the under surface of the lamina, so that the long 



M.B. D 



