36 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
later. Two possible explanations of the exceptions can be given. Either 
rainfall is not the exciting stimulus, or the plants can not be aroused by 
heavy precipitation whenever it occurs. 
If the hypothesis of Rutgers and Went is correct, namely, that bud 
development to a definite point, the same in all individuals, determines 
simultaneity of flowering, then those exceptions in which no flowering occurs 
eight days after a heavy rainfall are explainable on the basis that there 
were no buds awaiting further development at that time. Perpetual 
flowering is hardly to be expected. Those other but fewer exceptions in 
which flowering does occur but without heavy rainfall on the eighth pre¬ 
ceding day are explainable only on the assumption that rainfall is not the 
determining factor. Possibly we are forced to come to this conclusion. 
But it seems very likely that, whatever the stimulus is, it commonly occurs 
with heavy precipitation. Burkill believes that changes in temperature 
probably determine the time of occurrence of flowering. Since changes in 
temperature usually accompany heavy precipitation and may take place 
without rainfall, it is quite possible that temperature is the exciting stimulus. 
As to what the factor actually is we are quite ignorant. 
The theory that the time of simultaneous flowering of Dendrobium 
crumenatum is determined by a climatic factor commonly associated with 
heavy rainfall and occurring eight days in advance of the day of flowering 
is supported by the Buitenzorg data so far presented. Most of the excep¬ 
tions considered can be more or less satisfactorily brought into general 
agreement with the theory. There is, however, another and possibly 
insurmountable objection to the theory. 
When two or more successive flowering days occur very close together, 
it is difficult to conceive of one (the second) lot of buds responding to the 
requisite stimulus eight days in advance of the flowering date and failing 
to respond to an identical stimulus which only two days earlier started the 
first lot of buds on the final stage of their development. A striking example 
of this was pointed out to me by Dr. Smith of the Buitenzorg Herbarium, 
and is recorded in the table under the dates of Aug. 24 and 26, 1919. 
Considered separately, these two flowering dates fit into the theory 
perfectly. In both cases the day of flowering was preceded by a heavy 
rainfall on the eighth (or ninth) day previous to anthesis, and this heavy 
precipitation ended a long dry period. But when the two flowering days 
are regarded together, one wonders if the physiological state of the buds is 
quite so nicely adjusted as to leave some of them unresponsive on Aug. 24th 
to the same kind of stimulus which two days later, on Aug. 26th, arouses 
them to further development. 
Such a condition is not to be regarded as impossible. In this connection 
it is worth noting that, while the same plant may produce blossoms on 
two successive flowering dates, no one shoot of any plant bears blossoms 
on both of two successive flowering days; that is, if a shoot of a plant 
