22 A TEXTBOOK OF GENERAL BOTANY 
leaves which fold together at night. We know also that the 
younger parts of many plants bend toward the light. Numerous 
small plants which live in water have the power of moving from 
place to place, just as is the case with animals. 
.. Irritability. The property of responding to external stimuli 
is known as irritability. A good example is afforded by the ef- 
fect of various stimuli, such as heat or chemicals, on the move- 
ment of protoplasm. Moderate heat increases the rate, while low 

Fig. 12. Drawings, made at fifteen-minute intervals, of a cell from a hair 
of a squash plant 
Note the changes in the arrangement of the protoplasm. (x 180) 
temperatures decrease it. Some chemicals accelerate it, while 
others have the opposite effect. The effect of a stimulus on proto- 
plasm may be evident in the movement of a whole organ, as when 
the leaves of a sensitive plant close as the result of contact or 
of heat, or when a stem or a leaf bends toward the light. 
Vacuoles. The larger part of the space within a mature cell — 
is usually occupied by a vacuole. This is a clear space which 
contains water with small quantities of material dissolved in it. 
The principal use of vacuoles is, by enlarging the cell, to increase 
its absorbing surface ; besides this, water, food, or waste material 
can be stored in them. Vacuoles are not present in very young 
cells, but as the cell increases in size small vacuoles appear and 
then gradually enlarge. As they increase in size they coalesce 
