AND IRRITABILITY OF SENSITIVE PLANTS 193 



most abundant constituent of the flora of the Amazon is Leguminosae. " 

 (p. 258) "Sensitive plants, here called Sleepy Plants (Dormideiras), 

 are here so common that almost e\-erv' day I scratch my fingers or my 

 shins against some thorny member of the group. " 



In reference to Mimosa, Wallace (46, p. 262) says: "Among the 

 more humble forms of vegetation that attract the traveler's notice none 

 are more interesting than the sensitive species of Mimosa. Where a 

 large surface of ground is covered with these plants the effect of walking 

 over it is most peculiar. At each step the plants for some distance 

 round suddenly droop, as if struck by paralysis, and a broad track of 

 prostrate herbage, several feet wide, is distinctly marked out by the 

 different colour of the closed leaflets. The true sensitive plants are all 

 low-growing herbs or shrubs with deUcate foliage, which might possibly 

 be liable to destruction by herbivorous animals, a fate which they may 

 perhaps escape by their singular power of suddenly collapsing before 

 the jaws opened to devour them." 



In the Old World tropics many Leguminosae are found, but not so 

 abundant as in the New World. Some genera, like Mimosa, Cassia, 

 Desmoditim, Acacia, Prosopis, Parkia, Cynometra, Copaiba, Bauhinia, 

 Sophora, Crotalaria, Indigofera, Astragalus, Aeschynomene, Vigna and 

 Dolichos have representatives in the tropics and sub-tropics of both 

 hemispheres. Comparatively few genera are restricted to the Old 

 W'orld. 



From the general survey on the distribution of the Leguminosae 

 it will be noted that a few members extend to the Arctic regions, e.g., 

 Lathyrus maritimus; several species of Desmodium reach their southern- 

 most limit in Patagonia. More representatives of the family are found 

 in the temperate zones, with a gradual increase in the number of genera 

 and species toward the tropics where the leguminous plants flourish 

 in great abundance. 



The increase of temperature is correlated with an increase of the 

 number of plants of this family and with the gradual advance in the 

 sensitivity of the different members that show this phenomenon. Nearly 

 the entire family shows night sleep of leaves. A considerable number 

 move paraheliotropically. Other sensitive responses have been evolved 

 gradually. A considerable number of leguminous plants of the tem- 

 perate regions respond to mechanical, electrical, and chemical stimuli, 

 but the movement is sluggish and not propagated for any great distance 

 from the center of the stimulation. The plants that show the best 



