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- 
less as cytoplasm unless _they develop dincaieell ota leds chow 
pigments. Second, there is the chloro- ing wall (w) and layer of 
phyll which is the green pigment that protoplasm (p) containing 
saturates the plastid, which is then known the nucleus (x) and chloro- 
as a hloroplastid or by the shorter term Plasts (c). » is the large 
chloroplast. In the higher plants the pentral vacuole: 
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 w:th dirt becomes yellow. On the other hand, when 
Fre. 233. — Chloren- 
