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 



