AND IRRITABILITY OF SENSITIVE PLANTS 195 



leaves of the sensitives, and these further explain why so many of the 

 Leguminosae have acquired this phenomenon of leaf movement. 



IVIany genera probably once sensitive, now, owing to colder condi- 

 tions as the plants were farther and farther removed from the tropics, 

 have non-sensitive, or slightly sensitive species. Other genera that are 

 distributed through colder regions and that are sensitive to day and 

 night changes have not acquired sensitivity to ordinary stimuli, or may 

 have lost sensitivity. 



Oxalidaceae 

 This family in its distribution and sensitive relations, as well as in 

 structural details, shows many similarities to the Leguminosae. The 

 family is much smaller than the one previously described, comprising 

 seven genera with about 250 species. The Oxalids are found in both 

 hemispheres, with the largest number in the Western Hemisphere, 

 extending from Northern Canada to the southern extremity of South 

 America. In the Old World the range is from northern Europe and 

 Siberia to New Zealand in the Southern Hemisphere. 



Oxalis Acetosella is the most northern representative of the genus 

 and of the family, being found in northern and middle Europe, eastward 

 through Siberia and south to the Himalaya Mountains. In the New 

 World, this plant is distributed throughout north central Canada, in 

 eastern Canada, south in the mountains to North Carolina. It has long 

 been a favorite plant for study, since it shows very striking nyctitropic 

 movements of its leaflets. It also shows slight paraheliotropic response, 

 and response to other stimuli. 



In our region, seven species of Oxalis are found; two of these — 0. 

 corniculata and 0. strida — are very common. The leaflets of the dif- 

 ferent species move nyctitropically and paraheliotropically, but exhibit 

 very sluggish movements when stimulated by mechanical, chemical, 

 or other means of stimulation. 



In the sub-tropics there is a marked increase in the number of species 

 of Oxalis, and other genersi— Bio phy turn and Averrhoa—a.re also repre- 

 sented. Biophytum dendr aides is found from central Mexico south- 

 ward. Averrhoa is distinctly a tropical genus, but both of the species 

 extend into some of the sub-tropical regions of Asia. 



By far the largest number of species of Oxalidaceae is found in the 

 tropics of both hemispheres, with the majority in the New World. In 

 South America the genus Oxalis is represented by more than 200 species, 



