192 BIOLOGICAL LECTURES. 



the base of the termhial leaflet, the latter will move through 

 a large angle, while the side leaflets will not at all or only 

 slightly participate. A bit laid similarly on one of the side 

 leaflets will make it move independently of the others. By 

 conducting control experiments with weights equal to the 

 particles of ice used, it can be proved that cold stimulus and 

 not weight of the particles is the determining factor. 



As with DioiicBa, so here a drop of hot water and the appli- 

 cation of a heated wire excite to contraction, but instead 

 of touching analytically on these I should prefer to dwell 

 on the question of parathermotropism. All my observations 

 go to show that this is largely due to heat stimulation 

 of the irrito-contractile cells, acting steadily for some minutes 

 on these, and that the movements can be hastened or arrested 

 by a rise or fall of a few degrees of temperature. Moreover, 

 it seems undoubted that the soil temperature has much to do 

 with the amount of contraction that each leaf undergoes. It 

 is not difficult to understand why this should be. Given a soil 

 on which Oxalis is growing that is covered by a pretty close 

 herbage, above which the plant rears its tuft of leaves ; the 

 soil protected from the sun's rays, and retaining its moisture 

 by reason of the roots that spread through it, will furnish 

 currents of cool water that will constantly rise into tlie leaves 

 of the plant. ICvaporation of the surplus moisture thus passed 

 into the leaves will further lessen the temperature of the 

 tissues. I may be allowed to quote only one set of statistics 

 in support. On a close, dull, but rather warm day, with 

 the temperature at 25° C. in the shade, several plants were 

 studied that grew under the shade of trees, and amid an 

 abundant herbage. The ground temperature was 23° C. All 

 the leaflets were either fully expanded or inclined slightly 

 upwards from their point of union. Another set grew on a 

 rather moist bank and were slightly overshadowed by a tree. 

 The thermometer placed alongside the plants, and like them 

 exposed to a 'slightly higher tlian the true shade heat, registered 

 27° C, while the surface of the soil below registered 25.5° C. 

 The leaflets were flat or faintly inclined downwards. Alongside 

 a third set, about two yards from the last, the thermometer 



