RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 9 



The first requirement, then, is that the present plant should live 

 long enough to accumulate by absorption from the soil a quantity of 

 phosphorus which may be concentrated in this one seed, or ten 

 thousand seeds, as the case may be. 



To accomplish this object, it is rather evident that a large amount 

 of water must be absorbed and disposed of, with a resulting deposit 

 of phosphorus and other solids as the water is evaporated. Even 

 then there must be a strong tendency for such solids, if retained in 

 solution, to diffuse back to the roots and into the soil. Not denying 

 the possible ability of the plant to trap and hold phosphorus, or any 

 other needed substance, at the point where needed, it seems necessary 

 to call into play some other physical force to effect this concentration. 

 The only other possible force is the electromagnetic affinity of energy 

 for matter and of matter for energy. The ability of the plant to 

 concentrate the essential inorganic substances in the best-lighted 

 parts of its structure may thus be explained. 



In other words, the requirement of plants for light is primarily a 

 requirement for a concentration of essential substances needed for 

 reproduction. But light can only be obtained where there is com- 

 petition through growth. To insure the necessary amount of light 

 the individual plant is required to keep its head at least up to the level 

 of the competitors, and the plant which becomes dominant is most 

 certain to reproduce. Possible differences between plants, in their 

 ability to make use of different kinds of light, need not be discussed 



here. 



So, then, reproduction requires light, the need for light calls for 

 growth, and growth in turn is possible only through the action of 

 light in photo-synthesis, or the creation of new organic matter by the 

 combination of water and carbon dioxide. 



This necessary combination of water from the soil and carbon 

 dioxide from the air can be effected only by exposing the cells con- 

 taining water to the air, so that the carbon dioxide may be absorbed 

 by these cells. The important feature ecologically is that such ex- 

 posure inevitably results in considerable losses of water; and even 

 though the cells so exposed may be somewhat protected, it is evi- 

 dent that carbon-dioxide absorption and water loss must, in a given 

 plant, run about parallel, both being controlled by the size of the 

 stomatal openings. The actual water loss, of course, will vary ac- 

 cording to the dryness of the air, the concentration and vapor 

 pressure of the contents of the exposed cell, and the intensity of the 

 light in which the operation is performed. Thus a plant capable of 

 making use of diffused light, or largely of the so-called actinic rays, 

 may function with less water loss than one exposed to the full heat- 

 ing effect of sunlight. 



