STIMULATION AND ITS EESULTS 371 



captured prey. Less conspicuously purposeful are those 

 changes in metabolism which are brought about in conse- 

 quence of interference with the supply of food or oxygen, 

 but even here evidence of purpose can be found if 

 sought for. 



To understand the purposeful changes in the behaviour 

 of plants when they encounter modification of their sur- 

 rounding conditions, we may consider briefly the nature of 

 their environment. In the case of an ordinary terrestrial 

 plant we find it to be as follows. The root system is 

 embedded in the soil, among the particles of which the 

 young root branches ramify as they grow, and to them the 

 root-hairs become firmly attached ; the soil undergoes 

 usually only comparatively small changes of temperature, 

 but is subject to a great deal of variation with respect to 

 the amount of water it contains and the distribution of 

 that water ; it is composed of various materials, partly 

 organic, partly inorganic, many of which are eagerly sought 

 for by the plant, but others of them are of no use to it ; 

 of the former, some though valuable are not in a suitable 

 condition for absorption. The stem rises vertically into 

 the air and bears its branches and leaves ; the air sur- 

 rounding them contains a varying amount of aqueous 

 vapour, together with a fairly constant quantity of carbon 

 dioxide. The sub-aerial portion is subjected to the alter- 

 nation of day and night, involving almost constant changes 

 of degree of illumination, together with varying direction 

 of the incident rays. During these times it meets with 

 considerable variations of temperature and moisture as 

 well as light. The whole plant is constantly acted on by 

 the force of gravity. The subterranean portions are less 

 affected by light, but they nevertheless receive a certain 

 amount through the crevices between the particles of the 

 soil, which varies from time to time both in amount and 

 in direction. The environment, though to a certain extent 

 constant, is nevertheless continually varying in these 



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