250 



LEAVES 



In leaves having an oblique or vertical position, palisade tissue 

 may be present also on the lower side. The spongy tissue, having 

 fewer chloroplasts and so characterized on account of its loose 

 structure, occupies the region between the palisade tissue and 

 lower epidermis or the region between the pahsade tissues when 

 there is a lower palisade tissue present. It consists of cells irregu- 



FiG. 232. — Cross section of a Tomato leaf, e, upper epidermis; c, cuti- 

 cle; p, palisade cells; s, spongy cells; d, lower epidermis; st, stoma; g, guard 

 cells of the stoma; h, stomatal chamber; v, vein; w, parenchyma sheath of 

 the vein. The small bodies shown in the pahsade and spongy cells are the 

 chloroplasts. 



lar in shape and so loosely joined as to provide a system of air 

 spaces which extend in all directions reaching from the stomata 

 into the palisade tissues. In function, which is the manufacture 

 of food, the palisade and spongy mesophylls are identical. 



Structurally chlorenchyma cells are well adapted to their 

 function. Their thin cellulose walls permit water and sub- 

 stances in solution to pass in or out readily. They have proto- 

 plasm, which, as in all living cells, is the substance endowed with 

 Ufe and, therefore, able to regulate its activities. The cytoplasm 

 (the name applied to all of the protoplasm except the nucleus) 

 only partially fills the cell cavity, forming only a peripheral 

 layer. In this peripheral layer the nucleus and also the chloro- 

 plasts are located. Such an arrangement of the protoplasm 



