42 A TEXTBOOK OF GENERAL BOTANY 
this condition is said to be plasmolyzed. If the plasmolysis has 
not gone too far, the removal of the concentrated solution and 
its replacement by a weak solution will allow the cell contents 
to absorb water and resume their original distribution. When 
fresh-water plants or the roots of land plants are placed in salt 
water, they are killed by plasmolysis. 
Water may pass from cell to cell by means of osmotic pressure 
or the hydration of colloids. When a cell which is exposed to 
the air is losing water, the concentration of the dissolved sub- 
stances in it is increased, so that it tends to draw water from a 
neighboring cell. This cell in turn may draw water from another 
cell.- In this way water may be drawn from cell to cell until the 
source of supply is reached. 
A semipermeable membrane may be of such a nature that some 
dissolved substances will pass through it readily, while others 
will do so slowly or not at all. Some dissolved substances pass 
more readily than others through the semipermeable membranes 
of plant cells. 
PHOTOSYNTHESIS 
Definition. Photosynthesis is the formation of sugar from carbon 
dioxide and water in the presence of light; it takes place only 
where there is chlorophyll, which is found in the chloroplastids. 
Source of materials. The water used in photosynthesis is ab- 
sorbed by the plant roots and then carried to the leaves. The car- 
bon dioxide diffuses from the atmosphere through the stomata 
and then through the intercellular spaces. Carbon dioxide can- 
not enter the cells as a gas, but water from the cells permeates 
the cell walls of the chlorenchyma, and the carbon dioxide gas 
goes into solution in this water. As the water in the walls is 
continuous with that in the cells, the carbon dioxide in solution 
in the cell walls diffuses into the cells and finally reaches the 
chloroplastids. 
Process. Photosynthesis is a very complex process concerning 
the details of which we have very little actual information. The 
first stable product seems to be some form of sugar. The various 
