446 © BOTANICAL GAZETTE [pone 
General discussion 
In order to maintain existence, it is necessary for an organism 
to fulfil the fundamental requirements of life; it must be able to 
take in food; it must be able to digest its food; it must be able to 
oxidize or otherwise rearrange its substance to obtain energy; it 
must be able to eliminate its waste products; and it must be able 
to perpetuate its kind. Further, it must be able to’ perform all 
these functions in its particular, individual environment. As 
these individual plants cannot migrate, they must be able to accom- 
modate themselves to the changing environmental conditions or 
die. That they flourish from year to year in healthy condition is 
unquestionable evidence that they are able to cope with their 
environment. Their ability to invade genetically lower associa- 
tions of plants indicates that they are thriving rather than just 
merely existing in their habitat. 
Although a living plant is always the expression of the inte- 
gration of environmental and hereditary factors, the most important 
single factor in the environment is the physiological water supply. 
The modifications of plant structure which lead to the conser- 
vation of the water supply are termed xerophytic adaptations or 
xerophytic reactions. The presence of xerophytic adaptations does 
not necessarily predicate that the amount of water used by the 
plant is relatively small, but that the ratio of the amount used to 
that which the plant obtains tends to become less than unity . 
Some so-called xerophytic plants use as much or more than ordi- 
nary mesophytic plants, as Groom (20) found was the case with 
Larix decidua. They are xerophytic, however, because they 
cannot absorb a large amount of water in proportion to that which 
they could otherwise transpire. : 
This is particularly true in the summer, when plants have their 
transpiring organs. The loss of leaves during the winter is quite 
rightly regarded as a xerophytic adaptation. The bog ericads which 
were investigated, however, retain their leaves during the winter. 
This opens at once the question, are these plants xerophytes because 
of their summer or their winter environment? As it may be safely 
assumed that the evergreen habit is hereditary in these ericads, the 
reaction to the environment necessitates the xerophily. 
