CHIEF ENVIRONMENTAL CONDITIONS. 143 



4. DURATION ASPECT OF CHEMICAL CONDITIONS. 



Since the chemical environment of the plant is effective through con- 

 trolling the internal chemical conditions, and since such control is 

 manifested by outward and inward diffusion of material, it follows that 

 any given change in environment may produce the response of changed 

 activity in the plant only after the lapse of an adequate time period. 

 Diffusion of material through water requires considerable time in every 

 instance. Also, the physiological processes of the plant can produce 

 material transformations only in proportion to the length of time 

 during which they are operative at their different velocities, these 

 velocities being in part controlled by internal chemical conditions. 



The study of this feature of the chemical relation has just begun, and 

 it surely demands much attention. What may be the effect upon the 

 final result of a plant's activity, of frequent fluctuations in the chemical 

 nature and intensity of the surroundings, we are unable as yet to 

 surmise. 



VI. MECHANICAL CONDITIONS. 



1. GENERAL CONSIDERATIONS. 



All environmental conditions that are effective to influence plant 

 activity through pressure of material en masse are to be classified 

 as mechanical. From this point of view it matters not how the pres- 

 sure may have originated or whether actual molar motion be produced. 

 We should thus consider the flattened root which is confined within 

 a rock-cleft, the one-sided development of trees growing in a wind- 

 swept mountain pass where the direction of air-movement is predomi- 

 nantly the same, the deformed branches, etc., produced by a heavy 

 fall of snow, and the fantastic forms often exhibited by shrubs such as 

 the hawthorns when continually browsed by animals, as all due to 

 mechanical conditions. A mass pressure applied from without may 

 merely hinder expansion of tissues, it may tend to compress certain 

 parts or organs, or it may actually bring about a tearing or cutting of 



the tissues. 



One special form of mechanical pressure, which is of basic im- 

 portance in plant growth, is definitely due to external conditions, but 

 is first developed within the plant-body. We refer to the asyni- 

 metrical pressures produced in tissues and cells by the action of gravi- 

 tation. Here an influence, still practically unknown excepting in 

 its most general aspects, not a simple pressure of body upon body nor 

 a diffusion of material, nor yet any form of energy transfer that is 

 apparently at all immediately related to light, heat, and electricity, 

 reaches from the external world through the periphery of the plant 

 and largely controls certain forms of cell activity. In this we may 

 be fau-ly certain that the material condition within the organism, 



