AND IRRITABILITY OF SENSITIVE PLANTS 191 



species of Trifolmm, Lotus, Astragalus, Phaseolus, Strophostyles and 

 Petalostemum, and also additional members of the three genera previously- 

 mentioned, is at a somewhat lower latitude. The leaves of these forms 

 show the day and night novements, some of them, like Trifolium, to a 

 very marked degree. Darwin (p. 349) examined eleven species of 

 Trifolium and found that nearly every one showed nyctitropic move- 

 ment in its leaves and also in the cot}dedons in some of them. 



The paraheliotropic response is well illustrated in some of these 

 more northern representatives of the family. In Strophostyles helvola, 

 the trailing wild bean, of eastern North America from Quebec south- 

 ward, the leaflets of each of the tri-foliate leaves turn their edges toward 

 the sun when the light is intense and the temperature approaches 32° C. 



In the warm temperate regions an increasing number of Leguminosae 

 show sensitivity. In a climate such as ours, with its warm, or almost 

 sub-tropical summers, with temperatures not unusually during the day 

 of 32 to 40° C, or even higher, and generally with quite a sudden drop 

 of temperature after sunset, a marked effect on the vegetation is shown. 



When there is a drop in temperature, it means that some plants 

 must provide against the too rapid radiation of heat. This provision 

 the leguminous plants show in their capacity to close the leaflets at 

 night. Practically all the plants of this family in our region show this 

 phenomenon. Of those which are native, or introduced here, are species 

 of Cassia, Gleditschia, Gyvinocladus, Lupinus, Phaseolus, Medicago, 

 Melilotus, Amorpha, Tephrosia, Robinia, Astragalus, Aeschynomene, 

 Desmodium, Clitoria, Apios, Strophostyles, Pueraria, Amphicarpaea, etc. 



All of these are sensitive to nyctitropism, and nearly all to para- 

 heliotropism in that the leaflets turn their edges toward the sun when 

 the light intensity gets above a certain optimum which varies with the 

 different plants. Many of these plants also are sensitive to mechanical 

 and other forms of stimuli. 



Nyctitropic response is very well seen in the genus Cassia (Plate 

 LVIII, Fig. 1-2), that is tropical in its distribution and is represented here 

 by three outlying species. Night sleep of leaves is beautifully shown 

 by the arborescent members mentioned above, — Gleditschia, Gymnoda- 

 dus and Robinia. The last of these, the black locust, shows the pheno- 

 menon to good advantage. Shortly before sunset the leaflets, which 

 during the day, if the temperature does not exceed 32° C, are in a 

 horizontal position, begin to sink until in an hour or more they are back 

 to back, having described an angle of 90°. The day position of leaflets, 

 when the temperature rises to 32°-35°, or higher, is such that the edges 



