BARK— LEAF. 



159 



390. The Epidermis, or skin, is a tissue of thin, empty 

 cells investing every part of the higher plants, except 

 the spongioles of roots and the stigma of the pistil. The 

 Thallogens, and submerged water-plants among phanero- 

 gams, have no epidermis. 



In the young cherry stem the epidermis is a colorless membrane 

 which readily peels off in transverse 

 rings. On old trunks it is displaced 

 by the corl<y baric. Its. function is 

 to prevent too rapid evaporatioii of 

 the juices of the plant. 



391. The leaf has its origin 

 in both wood and bark ; the 

 fibres of which, vertical in the 

 stem, turn outward horizontally 

 in the leaf, forming the ribs and 

 veins. 



These are beautifully seen in skel- 

 eton leaves, from which the pulp has 

 been removed by maceration. In 

 endogenous leaves the long, parallel 

 veins sometimes have cross-veins, or 

 venules. In the Lattice - leaf of 

 Madagascar (Fig. 231) the pulp, or parenchyma, is often wanting 

 between the venules ; this handsome water-plant (submerged) gets 

 its specific name fenestralis (L. fenestra, window) from this circum- 

 stance. 



Fig. 231. — Lattice-leaf {Ouvirandra 

 fenestralis) in flower; If. a foot long, 

 a, young fl., spike enclosed in a coni- 

 cal spatlia ; b, spatlia removed ; c, 

 single fl. on part of spike. 



392, Leaf-pulp is an expansion of the green (or middle) 



bark. It is usually in two 

 layers (Fig. 232) ; the upper 

 ' {ps) faces the sky ; the lower 

 [pi) faces the earth. 



The cells of the upper layer 

 stand endwise, closely compacted ; 

 those of the lower face lie length- 

 wise, loosely arranged, with many 

 air-chambers between them, which 

 communicate with the epidermis 

 and inhale air through its stomata 

 (Fig. 233). In submerged water- 



Fio, 232.— Vert. sec. of lily leaf (Mium 

 candidum)^ enlargi'd : es, epidermis of upper 

 surface ; ti, of under surface ; p«, upper, pi, 

 lower, parenchyma ; m, intercellular spaces ; 

 I, lacunes. 



plants there is no epidermis, and for the slight respiration they need 

 large lacunes (Fig. 215, D) are provided. 



