The Kinds of Work That Are Done by Plants 87 



are kept under water. Roots, in fact, drown as truly and in ex- 

 actly the same physiological way as do animals, and with only 

 this difference, that roots can stand immersion for hours or dajs, 

 while animals can endure it only for minutes. This explains 

 the need for drainage of wet soils; it is not that these have too 

 much water, but too little air. It explains also why the soil of 

 flower pots needs to be carefully drained, and the cause of the 

 failure of so many persons in the care of their house plants, which 

 most people keep too constantly wet. The very best treatment 

 for most potted plants is to give to the soil an occasional soaking, 

 and allow it to dry out pretty well in between tmies; the roots do 

 not mind the absence of air for some of the time if they can have a 

 sufficiency at other times. IMoreover this method of watering has 

 another great advantage over that of adding a little water more 

 frequently, in the far greater effectiveness with which it drives 

 out the foul air and ensures a fresh supply. 



Another economic phase of respiration is involved in the 

 popular belief that it is unhealthful to keep house plants in sleep- 

 ing rooms. It will now be plain to the reader that this belief is 

 correct. But in fact the danger is slight. The amount of carbon 

 dioxide given off in respiration by a square meter of leaf is only 

 about the three-hundredth part of that given off in the same time 

 by a person, and although buds and roots respire more actively, 

 it is hkely that a whole window-full of plants does not give off 

 one fiftieth of the amount that one person does. Or, it has been 

 stated thus, that all of the plants which could be crowded into 

 the windows of any ordinary sleeping room give off less carbon 

 dioxide to the air than would a tiny light kept burning over night ; 

 and nobody would consider this quantity injurious, especially 

 if the room were ventilated as it should be. Indeed, were the 

 respiration of the plants in a room not negligibly small, it would 

 obviously be unsafe for any person to camp out in a forest in 

 summer! 



We must now come back to the more technical aspects of res- 



