602 The Philippine Journal of Science 
rates of transpiration. It may be added that the length of the 
period during which the plants are exposed to the environmental 
conditions is unquestionably very important in determining the 
nature of such results as were obtained in the present study. In 
consequence of these considerations, it is evident that conclusions 
drawn from this study can be regarded as applicable only for 
nonsolution conditions that are not greatly different from those 
encountered in these experiments. As has been mentioned, these 
conditions were, in a general way, those of a greenhouse in the 
winter and spring in the temperate region. 
Aside from the conditions of environment cin is of course 
another set of conditions that is very influential in determining 
plant activities. This is the group of internal conditions, which 
are effective from within the plant body itself. All that can be 
done toward the control of these internal conditions must be 
accomplished either by employing for the experiments plants 
that are as nearly alike internally as possible, or by subjecting 
the available plants to thoroughly controlled environments for an 
adequate period to make them alike. In this work, as in most 
experimentation of this kind, considerable care was exercised to 
employ plants, for the different cultures of any series, that were 
very nearly alike in size and general appearance. This method 
is clearly not entirely satisfactory, but it is about as good as 
the present state of our knowledge permits. Of course it is 
evident, just as in the case of the environmental conditions, that 
the conclusions of this study can be interpreted only with re- 
ference to the particular, initial, internal conditions of the plants 
used. It is not to be expected that the results would have 
been quite the same if some other species or variety of plant 
had been used, or if the plants had been introduced into the 
experiments at a different stage of their development. 
While the somewhat complicated considerations just expressed 
may make such problems as those dealt with in this study 
seem very difficult, or even practically impossible of solution, 
from our present point of view, yet, physiological progress seems 
to lie in the accumulation of such partial answers to these ques- 
tions as may be obtained experimentally, and in the careful 
correlation of these partial answers as they become available. It 
was with this idea in mind that the study here reported was 
carried out. 
