Plants as Affected by Insuffictent Water. 131 
tuber can no longer divide, hence growth is limited to the 
younger parts, i. e., the vicinity of the buds or eyes, and 
these therefore grow out into unshapely protuberances. 
The formation of 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 knobbi- 
ness than others. In varieties 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 moisture by 
ventilation and the use of water. Out of doors, we guard 
against excessive moisture in the air by giving plants suf- 
ficient room to favor the circulation of air between them. 
The latter precaution is especially important in orchard 
planting, since several fungi that prey upon fruit trees, as 
the apple scab (328) and pear blight (323), flourish in a damp 
atmosphere. 
B— THE PLANT AS AFFECTED BY INSUFFICIENT WATER 
228, Insufficient Moisture, in the Air, Causes Exces= 
sive Transpiration (75), which reduces the tension of the 
cell-walls and thus retards growth (635). It also tends to 
clog the leaves with useless mineral matters, causing their 
premature death, and favors the development of certain 
fungous parasites. The effects of insufficient moisture in 
the air are often very noticeable upon plants kept in living 
