370 VEGETABLE PHYSIOLOGY 
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 alterna- 
tion of day and night, involving almost continuous 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 
respects, so that no two plants are situated exactly simi- 
larly though they may be growing side by side. 
