THE PLANT AS AFFECTED BY WATER 133 



following a drought during the ripening season. Parts 

 of the tuber that are still capable of growth, stimulated by 

 abundant water, resume such activity. But since cell 

 division is possible only in the parts containing protoplasm, 

 the mature cells of the tuber can no longer divide, hence 

 growth is limited to the jounger parts, i.e., the vicinity of 

 the buds (eyes), and these therefore grow out into un- 

 shapely protuberances. The knob consumes a part of 

 the starch previously stored in the tuber from which it 

 grows, hence knobby potatoes are poorer in food value 

 than smooth ones of the same lot. 



Certain varieties of the potato are more disposed to 

 knobbiness than others. In varieties normally free from 

 it, the planting of knobby seed tubers probably does not 

 tend to increase the inclination to knobbiness. 



226. Air moisture. — Excessive moisture in the air is 

 injurious to plants, since it tends to hinder normal tran- 

 spiration (74) and favors the growth of certain fungous 

 parasites (321). In the greenhouse, we control the at- 

 mospheric moisture by ventilation and care in the use of 

 water. Out-of-doors, we guard against excessive moisture 

 in the air by giving plants sufficient room to favor the cir- 

 culation of air between them. The latter precaution is 

 important in orchard planting, since several diseases 

 that prey upon fruit-trees, as the apple scab (328) and 

 pear blight (323), flourish in a damp atmosphere. 



THE PLANT AS AFFECTED BY INSUFFICIENT WATEB 



227. Excessive transpiration. — Insufficient moisture 

 in the air causes excessive transpiration (74), which re- 

 duces the tension of the cell-walls and thus retards growth 

 (62). It also tends to clog the leaves with useless mineral 



