CELLULAR STRUCTURE OF LEAVES 



251 



places the chloroplasts around the cell wall where they are well 

 exposed to light, and provides a large central vacuole which 

 accommodates a large quantity of cell sap consisting of water in 

 which sugar, carbon dioxide, oxygen, mineral salts, and other 

 substances related to the activities of the cell are dissolved. 

 (Fig. 233.) Through the layer of protoplasm, the outer border 

 of which behaves as an osmotic membrane, the cell sap osmoti- 

 cally pulls in water from the veins or surrounding cells, and in 

 this way develops a pressure which dis- 

 tends and gives rigidity to the cell. Its 

 cells being rigid, the leaf is rigid and ex- 

 panded to the light. That this pressure 

 or turgor within the cells gives rigidity 

 to the leaf is shown by the fact that leaves 

 wilt when water is so scarce that the cells 

 can not maintain their internal pressure. 



The chloroplasts, usually oval in shape 

 in Flowering Plants, consist of two sub- 

 stances. First, the chloroplast has a body 

 which consists of cytoplasm denser than 

 ordinary cytoplasm and known as a 

 plastid. Plastids multiply by constrict- 

 ing into two equal parts, and are as color- 



,, , T FIG. 233. Chloren- 



less as cytoplasm unless they develop chyma cell of a leaf , show _ 



pigments. Second, there is the chloro- ing wall ^ and layer of 

 phyll which is the green pigment that protoplasm (p) containing 

 saturates the plastid, which is then known the nucleus (n) and chloro- 

 as a chloroplastid or by the shorter term p ^^ ^ the large 

 chloroplast. In the higher plants the 



chlorophyll is developed by the plastids and does not occur 

 except in connection with these bodies. Plastids are common 

 in all parts of the plant. In regions where they develop no 

 pigments, the formation of starch from the sugar present is their 

 chief function. They are even abundant in underground organs, 

 such as fleshy stems and roots, which store starch. 



The presence of chlorophyll depends mainly upon exposure 

 to light. That chlorophyll disappears in the absence of light is 

 well demonstrated by the fact that leaves lose their green color 

 when light is excluded for a time. Thus Grass under a board 

 or covered with dirt becomes yellow. On the other hand, when 



