47 2 Mason . — Growth and Abscission in Sea Island Cotton. 
It is important to observe that no relationship can be traced between 
the fluctuations in the daily rainfall and the rate of growth. The only 
marked check in the growth-rate occurred during a period of low evaporation 
which was as usual accompanied by daytime rain and little or no direct 
solar radiation. It cannot but be considered remarkable that the heavy 
daily application of water to Gp. I, and the isolation of the subterranean 
environment in Gp. IV, should have been without any appreciable influence 
on the daily growth-rate. On the whole, the results confirm the view that 
inhibition of growth in the main axis is due to a change in the aerial rather 
than in the subterranean environment. 
Water Relations and Daytime Growth. 
Reference has already been made to the fact that every previous 
investigation of the factors responsible for abscission has pointed to the 
importance of the plant’s water relations. Balls’s work, for instance, leaves 
no scope for doubt that under Egyptian conditions a pronounced water- 
deficit in the plant body is the main cause of shedding. He demonstrated 
that in the sunshine the growth of the cotton-plant is inhibited. This 
growth-inhibition while the sun was up he called the ‘ sunshine effect ’. 
He showed that it resulted from the increased tension due to the excessive 
rates of transpiration by shading a plant with a bell-jar, whereupon growth 
was resumed even in the sunlight. Lloyd demonstrated that precisely the 
same relation was shown by the cotton-plant in Alabama, where a cessation 
of growth or shrinkage during a portion of the hours of sunlight was noted. 
The results of the experiments recorded in the preceding sections suggest, 
however, that the growth-inhibition which precedes shedding in St. Vincent 
results from a check in the assimilatory activity of the leaves rather than 
from marked daily fluctuations in the aridity of the plant’s environmont. 
It may be stated, however, that one of the results of a marked daily 
water-deficit would be to close the stomata and thus inhibit carbon 
assimilation. 
In order to ascertain whether the ‘ sunshine effect ’ occurred under the 
very humid conditions under which cotton is grown in St. Vincent, the 
thirty-one plants of the second group were measured both shortly after dawn 
(6 a.m.-8 a.m.) and also towards dusk (4 p.m.-6 p.m.) ; the daytime 
interval between the measurements being therefore approximately 10 hrs. 
With a view to indicating how the growth during the day compared with 
that at night, the observed increments in growth have been expressed on 
a 24-hour basis. The results, which have been smoothed to four-day 
periods, are shown graphically in Fig. 10. The daily evaporation and 
rainfall, smoothed for a similar period, are also reproduced in the figure. 
Inspection of the graph discloses the fact that growth was inhibited to only 
a very small extent during the hours of sunshine. Some retardation in the 
