Feb., 1923] 
SEIFRIZ-CAUSES OF GREGARIOUS FLOWERING 
107 
The Gregarious Flowering of the Orchid Dendrobium crumenatum 
Gregarious flowering is characteristic not only of bamboos (and to a 
limited extent of the talipot palm), but also of the orchid Dendrobium 
crumenatum. 
Wherever a number of individuals of the orchid Dendrobium crumenatum 
occur within the same general locality, the plants flower simultaneously. 
The blossoms of every plant burst forth on the morning and wither in the 
evening of the same day. 
Among the specimens of Dendrobium crumenatum in the Buitenzorg 
Gardens in Java there are plants collected from nearly all parts of the 
Dutch East Indies, from Riouw (near Singapore), from Sumatra, Java, 
Borneo, Celebes, and Ambon (a small island at the eastern end of the 
archipelago). These plants, after being brought to Buitenzorg, all flowered 
on the same day, if they flowered at all. Yet in their native habitat the 
flowering periods of the plants do not at all coincide. Thus, orchids growing 
in the virgin mountain forests flower on different days from those in the 
lowlands. Plants growing at two stations but three kilometers apart may 
differ in their times of flowering by one or two days. But wherever their 
original home and whatever the date of flowering there, the plants, when 
assembled in one locality, flower simultaneously with each other and with 
the plants which have grown in that locality from youth. There is no 
other explanation here but that some external factor determines the exact 
time of flowering. The interesting question arises, What is the controlling 
external factor? 
Burkhill, from data obtained in the Straits Settlements, comes to the 
conclusion that “climatic conditions some eight days in advance of the 
flowering are a controlling factor” in the gregarious flowering of Dendrobium 
crumenatum (3, p. 405). 
The writer has recently (21) published data from Buitenzorg which 
support the conclusion of Burkhill. If the flowering dates of the orchid 
are compared, in a table, with the daily precipitation data preceding all the 
flowering dates, it will be seen that in the majority of instances the rainfall 
on the eighth day preceding each day of gregarious flowering is unusually 
heavy. Especially evident does this become when the totals of the 
precipitation figures for the respective series of days are compared. The 
total rainfall occurring on the eighth day previous to all the flowering dates 
is five ninths greater than that of the next highest. The data strongly 
support the possibility that heavy rainfall eight days in advance of flowering 
is the cause of simultaneous flowering of the plants. But several striking 
exceptions occur which force one to conclude that the stimulating factor 
which arouses the resting flower buds to further activity is not rainfall but 
some other as yet unknown factor (possibly temperature) commonly asso¬ 
ciated with heavy rainfall. 
At first thought, the gregarious flowering of Dendrobium crumenatum is 
