IRRITABILITY 223 



mum ) amount causing them to take place more rapidly or 

 more energetically. There are, therefore, minimal and opti- 

 mal quantities for the ptrvsical states and the chemical 

 processes in the living cell and the living organism, but 

 until water is sufficient in amount to become the vehicle of 

 mechanical forces, there cannot be said to be any maximum 

 quantity. The minimum and optimum vary with the or- 

 ganism. 



An organism will absorb water up to a given amount, 

 but it can absorb no more (see p. 110) because the sub- 

 stances which it contains do not offer the physical condi- 

 tions making further absorption possible. The absorption 

 of water gives rise to changes in the physical conditions pre- 

 vailing within the cells ( pp. 210-11 ) , the changes manifesting 

 themselves most conspicuously in an increase in volume or 

 pressure, or both. These physical changes directly affect the 

 living protoplasm. The greater pressure or larger volume 

 of individual cells enables them to exert greater mechanical 

 force upon their surroundings and to occupy more space ; it 

 enables the organ which they compose to expand or to come 

 into a more favorable position, etc. Besides these mechani- 

 cal effects of water, which influence the living protoplasm, 

 the greater or less quantity of water in the cell exercises a 

 direct physiological effect upon the activities of the cell. 

 When the plant or the cell needs water, its activities will be 

 reduced, within certain limits, in proportion. Conversely, 

 any increase, under these conditions, hi the amount of water 

 available will stimulate the plant or the cell. 



Water exerts two distinct influences upon the plant. This 

 can best be shown by examining the directive influence of 

 water upon growth. First, let the downward-growing pri- 

 mary root of a seedling growing in air not excessively damp 

 be exactly measured as to diameter and length by the eye- 

 piece micrometer of a horizontal microscope; then plunge 

 this root into water of the same temperature as the air and 

 note the increase in dimensions which promptly follows. 

 This is a purely mechanical effect, swelling. Again, let seed- 

 lings be grown in any apparatus which will expose the 

 roots to unequal amounts of moisture on opposite sides ; for 



