Plants as Affected by Excessive Water. 141 



the excessive absorption of water by the roots. To pre- 

 vent it, we start the plants by pulling on the stem suffi- 

 ciently to break a part of the roots. 



225. Knobby Potatoes are caused by a wet period 

 following a drought during the ripening season. Parts 

 of the plant that are still alive, stimulated by abundant 

 water, resume growth. But since cell division is possi- 

 ble only in the parts containing • protoplasm, the ma- 

 ture 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 grow 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 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. Excessive Moisture iu the Air is Injurious to 

 plants, since it tends to hinder normal transpiration 

 (74) and favors the growth of certain fungous para- 

 sites (321). In the greenhouse, we control the atmos- 

 pheric moisture by ventilation and care in the use of 

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

 ure in the air by giving plants sufficient room to favor 

 the circulation of air between them. The latter precau- 

 tion is important in orchard planting, since several dis- 

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

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



