196 STECKBECK— ON COMPARATIVE HISTOLOGY 



including the most sensitive members of this genus. Of those in cultiva- 

 tion such sensitive forms as 0. bupleiirifolia, O. scandens and 0. Ortgiesii 

 are native to the New World tropics. Comparatively few species of 

 Oxalis are found in the Eastern Hemisphere. The genus Biophytum 

 is essentially tropical in distribution in America, Asia, and Africa. B. 

 sensitivum, the most sensitive species of this genus that has been studied, 

 is found in both the Old and the New World. B. dendroides, scarcely 

 less sensitive than the above species, is indigenous to the American 

 Tropics. Averrhoa, a small genus, but including several species, e.g., 

 A. carambola and A. bilimbi, that have been studied quite extensively, 

 is found wild or cultivated in the tropics of both hemispheres. 



Practically all the tropical species that have been studied exhibit 

 irrito-contractility in their leaves. Not only are they sensitive to day 

 and night conditions, but when stimulated mechanically, chemically, 

 electrically or otherwise the leaflets close, dropping back to back, below 

 the plane of the horizon. The sensitivity varies for the different species, 

 and, as compared with the most sensitive members of the Leguminosae, 

 is less marked. 



In the Oxalidaceae, as in the Leguminosae, the range of distribution 

 extends from the Arctic Circle on the north to the colder temperate 

 regions on the south, with by far the largest number of representatives 

 in the tropics. 



The relative sensitivity shows a gradual increase from the species 

 found in the colder regions to those types that exhibit the highest degree 

 of sensitiveness and which are distributed throughout the tropics. 



The probable reasons why the tropical forms of this family have 

 acquired sensitivity that is in advance of those in the cooler regions are 

 the same as those brought out for the Leguminosae. The two families 

 suggest many similarities. As indicated, the range of distribution is 

 practically identical with the climax in the number of species in the 

 tropics for both groups. In both we find compound leaves (a few excep- 

 tions in the Leguminosae), that are provided with primary pulvini at 

 the bases of the petioles, a secondary pulvinus at the base of each leaflet, 

 and if the leaf is bi-pinnately compound, as in Mimosa pudica, a tertiary 

 pulvinus at the base of each secondary leaflet. Both families show 

 similar response to stimuli. 



In the Leguminosae and in the Oxalidaceae, the phases of stimula- 

 tion are alike. In both: (1) there must be an optimum environment in 

 order that response may be shown. Such environal factors as suitable 



