146 ENVIRONMENTAL CONDITIONS. 



Books on ecology may be consulted for many instances of mechanical 

 influences that increase the geographic range of the activities of 

 plants, these being usually and curiously described as adaptations by 

 which plants ''have come to be" fitted to growth in certain habitats, 

 rather than as environmental adaptations by which the habitats have 

 become fitted to support certain kinds of plants! 



Practically none of these considerations, however, pertain logically 

 to a study of the proximate or immediate conditions controlling plant 

 activities ; these mechanical agencies of transport are of only secondary 

 interest; they may be said to be only causes of causes. Thus the 

 immediate external conditions usually considered as causing the 

 germination of a seed must be the entrance of water, of oxygen, and 

 of heat, and the reason for the occurrence of these immediate condi- 

 tions is to be sought in the preceding mechanical transport of the 

 seed. Of course such secondary causes are of great importance, and 

 it is often practically impossible to approach nearer than these to the 

 real seat of the external control of plant processes. While superficial 

 and merely qualitative studies upon such influences have been fre- 

 quent, the deeper-going quantitative and comparative work upon 

 them remains almost entirely for the future. 



VII. INTERRELATIONS OF THE ENVIRONMENTAL CONDITIONS. 



The present section is appended here merely to emphasize a feature 

 of the discussion of environments that has already received some at- 

 tention at several points in the foregoing pages, namely, that external 

 influences are seldom or never singly effective upon plants. Three 

 considerations in this connection require a short treatment: 



(1) The more remote conditions of the external world, in bringing 

 about the occurrence of any given influence upon plants, usually 

 inaugurate other influences at the same time. Thus, ^^dth an increase 

 in the amount of soil-moisture, the permeability of the soil to oxygen, 

 the concentration of the soil solution in salts, etc., and the power 

 of the soil to retain or give up heat, are more or less profoundly altered. 

 With an increase in the intensity of impinging light comes also an 

 increased income of heat to the foliage, and consequent alterations 

 in aerial convection currents about the plant. 



(2) The same external condition usually influences the velocity of 

 more than one of the elementary component processes of the organ- 

 ism. Thus, phosynthesis, respiration, digestion, excretion, secretion, 

 growth, etc., are all greatly influenced by such fundamental environ- 

 mental relations as those of water, temperature, light, etc. 



(3) Since every elementary physiological process is thoroughly 

 bound up with many other concomitant processes, it follows that an 

 external change that alters only one process directly may indirectly 

 be the cause of alteration in many others. If the secretion process, 



