1918] 
DUGGAR AND BONNS—RATE OF TRANSPIRATION 173 
plant type and summation of conditions are factors of impor- 
tance seems a well-warranted conclusion, as will be developed 
below. 
The results may be discussed in three categories, in respect 
to the plant material employed. In the first type of material 
the usual mesophytie potted plant has constituted the experi- 
mental object; in the second, a plant of xerophytie surface 
modifications, Cyperus esculentus; and third, abscised leaves 
of castor beans. Without exception, the potted plants in the 
first eategory furnished consistent evidenee that under the 
conditions of our experiments increase in the rate of trans- 
piration occurs mainly, if not entirely, during the night in- 
tervals. There may be little or no change fn the rate of 
transpiration during the day intervals, and, to a considerable 
extent, at least, this is independent of slight changes in 
weather conditions,—some of the experiments having been 
conducted in bright sunshine, others in cloudy weather, and 
in still other cases different intervals in the same series have 
furnished varied conditions. Nevertheless, the fact that the 
night interval has invariably exhibited, in respect to the 
sprayed plants, a transpiration increase, makes it clear that 
in some way the sum of night and day conditions is respon- 
sible for the increased water loss. Attempts to increase or 
diminish the humidity in the greenhouse by flooding with 
water has not resulted in any indications which alone might 
explain the observed phenomenon. 
The greenhouse was subject to a rise and fall of tempera- 
ture from midnight to midday, amounting to from 7 to 15° C. 
It has repeatedly been noticed that under such greenhouse 
conditions seedlings exhibit the phenomenon of guttation to 
some degree, often to a very high degree. Now if it may be 
assumed that the potted plants experimented upon were sub- 
ject each night to conditions inducing guttation, or at least 
incipient guttation, this condition might be made use of to 
explain the phenomenon in the following way: A film of 
Bordeaux mixture on the surface of a plant in a state of gut- 
tation would probably act more or less as a bibulous surface, 
taking water directly from the interior of the plant, through 
