Mason. — Growth and Abscission in Sea Island Cotton. 467 
completely removed. Here, too, it will be seen, the first shedding wave 
(commencing December 2 6 ) followed five days after a period of heavy 
daytime rain and low rates of evaporation. The second wave, it will be 
noted, followed some five days after a change from very low to rather high 
evaporation rates. It will be remembered, however, that the probability 
exists that a very slight stimulus during this later portion of the flowering 
period may initiate shedding, and that even in the absence of a stimulus 
of any sort a certain amount of shedding is apparently inevitable. 
To sum up, the results suggest that heavy rainfall is not directly 
responsible for the augmented rates of shedding ; nor yet, it would seem, 
is daytime rain to be especially associated with the initiation of abscission. 
It will be observed, however, that daytime rain when accompanied by very 
low evaporation rates is followed by pronounced shedding. That daytime 
rain may have occasioned some of the shedding as a result of the destruction 
of pollen is very probable. A more fundamental cause than the absence 
of pollination must, however, be sought. For it cannot but be significant, 
as Harland ( 3 ) has pointed out, and as the results presented in the next 
section indicate, that the waves of bud- and boll-shedding tend to be 
synchronous. Moreover, the relatively small percentage of flowers which 
subsequently underwent abscission, in spite of daytime rain, during the first 
half of the flowering period clearly indicates how slight is the part played 
by this factor in bringing about shedding. 
The Growth-rate of the Main Axis and Environmental 
Factors. 
In order to determine how the growth-rate of the main axis was 
influenced by daytime rain and low rates of evaporation, which the results 
presented in the preceding section seemed to suggest as being the precursors 
of the augmented rates of boll-shedding, the growth-rate of the thirty-three 
plants which constituted the first group was determined daily, between 
9 and 11 a.m., until growth became so small that no significance could be 
attached to the daily variations. The measurements were made from the 
cotyledonary node to the terminal bud. In Fig. 8 the mean results of these 
measurements for a period of forty days are presented for comparison on 
the one hand with rainfall and evaporation, and on the other with the 
rates of bud- and boll-shedding. Inasmuch as some of the flower-buds were 
shed before the opening of the attendant foliage leaf, and consequently the 
day on which they were shed is not known, only those which were shed 
subsequently have been included in the results presented in Fig. 8. 
Inspection of the graphs betrays the somewhat remarkable fact that 
low rates of evaporation accompanied, as is generally the case, by daytime 
rain corresponded with a marked retardation in the growth-rate. The 
amount of evaporation on September 8 and 9 was not determined, as the 
