242 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



Physiological studies of the rate of growth of a plant are generally carried 

 out by measuring the part in question, either with a simple millimeter rule or 

 with special measuring apparatus. In experiments of this kind only the external 

 growth, or the enlargement of the plant, is measured, and the rate of enlargement 

 is determined for a definite time period and under a certain set of conditions. 

 Internal growth cannot be studied with a rule, it can be measured only by means 

 of the microscope, or by qualitative and quantitative analyses of the materials 

 found in the plant at different periods of its development. 



§2. Conditions Favorable to Growth— Growth of the cell is a result of the 

 activity of protoplasm, and a large number of conditions must be fulfilled in 

 order that it may take place. If a single one of the necessary external conditions 

 be absent, then growth ceases, and if the internal conditions necessary for 

 growth are not all fulfilled growth fails to occur in this case also, even though 

 all other conditions are favorable. 



N- 



12 3 4 



Fig. 97. — Different stages in plasmolysis of a cell. N, nucleus; V, vacuole. (After de Vries.) 



Turgidity is one of the internal conditions necessary for cell enlargement. 

 If a growing cell is placed in a 10 per cent, solution of sodium chloride, potassium 

 nitrate or sugar, it immediately begins to decrease in size (Fig. 97). At first 

 the cell wall and the protoplasmic membrane contract equally, but later, when 

 the cell wall can contract no more, the protoplasm still continues to move 

 inward, thus retreating from the cell wall. Finally, the entire contents of the 

 cell collect into a ball-like mass in the center of the cell,with the outer proto- 

 plasmic membrane on the outside. This process is known as plasmolysis, as 

 has been pointed out (page 114). If a plasmolyzed cell is placed in pure water 

 it enlarges and finally regains its original size and form. The external condi- 

 tions that produce these changes in cells are likewise effective in causing the 

 shrinkage of an animal bladder filled with weak salt solution, when this is 

 placed in a strong salt solution. The cell sap of plant cells is a solution of 

 various substances, which have an attraction for water. The osmotic pressure 

 produced in the cell when plenty of water is supplied results in the turgidity of 

 the cell. 



The enlargement of each cell begins with the stretching of the cell wall by 



