227 
Leaf Water-content , and Transpiration Rate . 
air, by stopping the fan and closing the ends of the flue with wooden 
shields. Also control plants have been kept outside the flue. It has not 
been possible to demonstrate any significant difference between the stomata! 
records of plants in still air and those subjected to a wind of velocity twenty 
metres per minute when other conditions are similar. 
Another possible source of stimulation of the stomata is the movement 
of the plants when weighings are made. In the case of Eupatorium 
adenophortim , at least, it was sometimes found that after a weighing had 
been made the stomata were less widely open than immediately before the 
recorder was disconnected, and that a few minutes elapsed before they attained 
their former aperture. This slight movement was, of course, quite distinct 
from any normal changes due to differences of illumination, & c., and was 
ascribed to shock. If care is taken in moving the plants, it is a simple 
matter to prevent any closure at all, and it is very likely that this response 
to shock is not shown by all plants. It is, however, a matter for precaution. 
Experiments on Stomatal Aperture and Transpiration. 
This series of some twenty-six experiments was carried out at various 
times of the year in a greenhouse and most of the species in the list given 
above were used. In some cases rooted plants were used and in others 
shoots mounted in potometers. In either case the plant was often made 
ready for the experiment overnight, in order that in the morning the experi- 
ment could be started before the occurrence of maximal values in the 
functions under observation. 
The results, as far as they concern the inter-relationship of stomatal 
aperture and transpiration, were distinctly discordant. In some experiments 
the graphs of relative transpiration and stomatal aperture ran closely 
parallel, a maximum in the case of one coinciding with a maximum in the 
other. On the other hand, in many experiments there appeared to be no 
correlation whatever between the two graphs. 
Below are given the results of two experiments of the series, representing 
extreme cases of agreement and discrepancy, respectively, between the 
graphs of stomatal aperture and relative transpiration. 
Experiment 12. The plant used was Helianthus tuber osits , a species with 
large stomata on both surfaces of the leaf and a high transpiration rate. Half- 
hourly measurements of the evaporating power of the air and of transpira- 
tion rate were made, and the ratio of the two gives numbers proportional 
to ‘ relative transpiration \ The quantity of water absorbed by the plant 
during each thirty minutes was determined by potometer readings, and 
changes of stomatal aperture were recorded by porometer determinations 
every ten or fifteen minutes. Table I gives the results, which are also 
expressed graphically in Fig. 1. 
