20 BULLETIN 1018, U. S. DEPARTMENT OE AGRICULTURE. 
gators (Klebs, 16, and Apple-man. 2) in being able to maintain almost 
any- plant in continuous growth throughout the entire year would 
seem to indicate that the importance of periodicity has been exag- 
gerated. 
It is difficult to interpret the behavior of Pima cotton as above 
described on the basis of an inherent periodicity, but it is not im- 
probable that physiological reactions are involved. It is a common 
observation among growers that cotton plants once stunted by water 
shortage are much slower in response when water is finally applied 
than plants which have not been so severely checked. A case of 
this nature came under the writer's observation at the Sacaton sta- 
tion in 1920. Plants growing in a sandy soil showed visible distress 
from water shortage about two weeks before the first application of 
water on July 7. In response to this irrigation the plants grew 
only 9 inches in height during the ensuing three weeks and re- 
quired two additional irrigations before complete recover}^, while 
plants on adjoining plats, which had shown no water stress, grew 
14 inches during the three weeks following the first irrigation and 
responded promptly to each subsequent" irrigation by making addi- 
tional growth. The fact that all external factors except soil moisture 
were closely alike for all the plants in these experiments and that 
there was a distinct difference in the promptness of response to this 
particular factor gives probability to the interpretation that the 
cause is internal, but is subject to remedy by control of the external 
factors. 
When we come to consider the adoption of means for remedying 
this maladjustment of the plant to its water supply, the most prac- 
tical line along which to work seems to be in modifying the condi- 
tions of the subterranean environment. The need for some such 
corrective measure seems especially imperative when dealing with 
heavy rich soils which cause a quick and vigorous response of the 
plants to irrigation water when applied by conventional methods. 
An experiment was conducted in 1919 by the writer and his co- 
operator, Mr. Warren H. Parker, in adjacent plats to those above 
described, in the effort to effect more perfect control of the plant 
growth. 4 The plants were grown in pairs of rows, with a wide 
middle 5^ feet between pairs and a space of 3J feet between rows. 
The water was run in deep furrows made in the wide middle (PI. IV), 
while the narrow middles were left dry after the planting season. 
The effect of the water when applied under these conditions was not 
noted in the behavior of the plants until five or six days later, and 
4 A double-row method of planting and irrigating Egyptian cotton has been outlined in 
connection with the single-stalk method of suppressing the vegetative branches, and 
tested in recent seasons at the United States Experiment Farm at Bard, Calif., near 
Yuma, Ariz. (6, 7, 8, 21). 
