Plants as Affected by Insufficient Water. 137 



cells of the tuber can no longer divide, hence growth is 

 limited to the younger parts, i. e., the vicinity of the 

 buds (eyes), and these therefore gro^^■ out into unshapely 

 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 potato are more disposed to knob- 

 biness than others. In vaiieties normally free from it, 

 the planting of knobby seed tubers probably does not 

 tend to increase the inclination to knobbiness. 



227. Excessive Moisture in the Air is Injurious to plants, 

 since it tends to hinder normal transpiration (75) and 

 favors the growth of certain fungous parasites (321). In 

 the greenhouse, we control the atmospheric moistnre by 

 ventilation and care in the use of Avater. Out of doors, 

 we guard against excessive moisture in the air by giving 

 plants sufficient room to fa\'or the circulation of air be- 

 tween them. The latter precaution is especially import- 

 ant in orchard planting, since several fungi that prey 

 upon fruit trees, as the apple scab (328) and pear blight 

 (823), flourish in a damp atmosphere. 



B — The Plant as Affected ky Insufficient Water 



228. Insufficient Moisture in the Air Causes Excessive 

 Transpiration (75), which reduces the tension of the cell- 

 walls and thus retards gi'owth (63). It also tends to clog 

 the leaves with useless mineral matters, causing their 

 premature death (126), and favors the development of 

 certain fui^gous parasites. The effects of insufficient 

 moistnre in the air are often very noticeable upon plants 



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