THE LEAF 73 



be seen in the leaf of the Aspen, or as shown more in detail in the 

 Privet (Fig. 48). 



The upper surface is covered by a continuous layer of epidermis, 

 composed of oblong cells with their outer wall thickened and cuticu- 

 larised, better seen in Fig. 52, p. JJ. The lower surface is similarly 

 covered by epidermis, but the continuity of its cells is here and there 

 interrupted by pores (stomata), which allow communication between 

 the outer air and the intercellular spaces within. Between these two 

 epidermal layers lies the tissue of the mesophyll. Towards the upper 

 surface its cells are arranged with some degree of regularity, frequently 



Fig. 48. 

 Transverse section of lamina of Privet [Ligustrum vulgar e). ab.ax. = the abaxial 

 or lower surface. The protoplasts are omitted on the left-hand side. u.e. = upper 

 epidermis. I.e. = lower epidermis. i.sp.= intercellular space. v= vascular strand. 

 sJ=stoma. ££ = palisade parenchyma, s.p. =spongy parenchyma. cry = crystal. 

 ( x 75.) The palisade parenchyma is poorly developed in the section of this shade- 

 loving leaf. Compare Fig. 47. 



in two layers. The cells of these layers are of oblong form, and stand 

 parallel with one another, the ends of the outermost layer abutting 

 on the epidermis. From their form and arrangement they are called 

 the palisade-parenchyma. Towards the lower surface the cells are 

 less regular in form and arrangement, and as the intercellular spaces 

 are very large and numerous this tissue is described as the spongy 

 parenchyma. The whole mesophyll is composed of thin-walled cells, 

 with living cytoplasmic lining, a nucleus, and very numerous discoid, 

 green Chloroplasts. To these is due the full green colour of the 

 leaf. The intercellular spaces, so conspicuous in the spongy paren- 

 chyma, are continuous though of smaller size between the palisade- 

 cells, and they connect with the stomata. A specially large space is 

 usually present opposite the pore of each stoma. The whole mesophyll 



