62 BOTANY OF THE LIVING PLANT 



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

 Privet (Fig. 45). 



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

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

 larised. The lower surface is similarly covered by epidermis, but the 

 cells are less regular, while their continuity 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 Ues the tissue of the mesophyll. Towards the upper surface 

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



Fig. 45. 



Transverse section of lamina of Privet {Ligusirum vidgare). ab-ax =thie abasial 

 or lower surface. Tlie protoplasts are omitted on the left hand side, ir.e— upper 

 epidermis- /.£. = lower epidermis, i.s^. =intercellQlar space. y=vascular strand. 

 s^=stoma. ^^=palisade parenchyma. s.^. = spong:y parenchjmia. cyy=a^rstal. 



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 cytoplasm, a nucleus, and very numerous discoid, green 

 chlorophyll-corpuscles. To these is due the full green colour of the 

 leaf. The intercellular spaces so conspicuous in tlie 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 



