204 BOTANICAL GAZETTE [APRIL 
evaporation curve, but in all instances the deficit is relatively slight 
when compared with that recorded in the former studies of this spe- 
cies. The fall of the transpiration curve in the afternoon was just 
as gradual and slow as the morning rise, by 7:00 P.M. or soon after, 
reaching approximately the same level as that recorded for 5:00 
or 6:00 A.M. preceding the first rise. 
The prominent features of the curves as found in this series of 
studies were a gradual rise, a low maximum which comes approxi- 
mately at noon, the absence of a conspicuous saturation deficit, 
and the gradual decline to the night rate of transpiration. In 
the records obtained on the dunes there was very generally a 
depression about midday, and the curve was frequently character- 
ized by a secondary rise developing a lower mode in the middle 
of the afternoon. This was followed by the evening decline. Such 
development of a secondary mode in the transpiration curves was 
not a feature of the records obtained on clay soils, but was observed 
in a few readings only, when it was attributed to the influence of 
special factors such as the rapid decrease in relative humidity fol- 
lowing a period of precipitation in the early afternoon (fig. 4). 
Effect of environmental factors upon transpiration 
The foliar transpiration stream undergoes various fluctuations, 
occasioned by the stimulus of certain factors which by temporarily 
exerting a dominating influence may bring about a pronounced and 
rapid response in the rate of water loss. 
PRECIPITATION.—Passing thunder showers usually caused the 
coefficient of transpiration to fall very rapidly. Fig. 4 shows the 
effect of a shower at stations H and J, these graphs being plotted 
from data secured on June 8. The general atmospheric conditions 
other than recorded in the graphs are indicated in the lower figure. 
It will be seer that at the two stations the same depression appears 
during the period of precipitation. The corresponding depression 
in evaporation, temperature, and relative humidity curves shows 
these factors all working together to produce the same result, 
namely, a lower transpiration rate. Immediately following the 
shower, which lasted one and one-half hours, there was a sudden 
and rapid rise in the transpiration curve, accompanied by a cor- 
