CHAPTER VI 



THE LEAF 



§ 19. Which cells of the leaf are the most essential ? 



A MICROSCOPIC examination of the leaf shows us that it is 

 mainly composed of green parenchymatous cells, which are so 

 arranged as to form a flat expanded surface. Parenchyma is, 

 as we have already stated, a tissue which consists of rounded 

 or polygonal cells, the diameter in various directions being 

 approximately the same, though the cells may sometimes be 

 elongated to form a cylindrical structure. The walls are of 

 moderate thickness, and the cells are, for a time at least, or 

 permanently, filled with protoplasm, in which the vital processes 

 of formation and transformation of organic matter take place. 

 The eells of the parenchyma are therefore the most important 

 organs of the plant ; all the other forms of tissue are only of 

 secondary importance. This is true of the thin- walled loosely 

 packed cells forming the merenchyma {e.g., the pulpy tissue 

 of fruits), which is poor in protoplasm, and also of the 

 sclerenchyma (hard tissue of nut-shells), the cells of which 

 resemble parenchymatous cells, but are very thick-walled and 

 entirely devoid of contents. The prosenchyma, too, consisting 

 of long, pointed thick-walled cells, such as form the wood and 

 the hard bast, is, functionally speaking, only of secondary 

 importance. All these elements have secondary functions, 

 and are not found in the simpler forms of green plants, such 

 as Alg« and Mosses. Those which are thick-walled serve 

 chiefly as protection and support for the parenchymatous cells. 

 The chief function of the green parenchymatous cell is the 

 production of new organic material {assimilation), of which we 

 have already spoken in passing. This formative process only 

 takes place under the influence of light. The carbonic acid 



