IRKrrO-COXTRACTILITY IX PLANTS. 



193 



registered 29.5° C. when exposed, and 27.5'^ C. on the ground. 

 Some plants that were exposed to the sun's heat grew on a 

 rather dry soil with scant herbage, and their leaflets were 

 deflected through an angle of 52° to 65°. Lastly, on a bit of 

 hard, dry, whitish soil, the exposed temperature alongside the 

 plants was 31° C, and on the soil 33° C. The leaflets of 

 the plants were as strongly deflected as during night-sleep. 



The amount of moisture, therefore, that is drawn from a 

 cool shaded soil or from a hot, dry soil may largely determine, 

 by its temperature, the stimulus given to the protoplasm and 

 the position that the leaflets are to assume accordingly. A 

 cooperating factor in the parathermotropic movement may be 

 a lack of sufficient moisture, which, in all sensitive plants 

 studied, causes flaccidity and a want of tone in the tissues. 



We shall have occasion in treating of other plants to speak 

 of the action of such chemicals as ammonia, carbonate of 

 ammonia, chloroform, ether, alcohol, etc., which all act as 

 excitants. 



Oxalis Dcppci is a large succulent species, commonly grown 

 now as an edging plant in herbaceous borders. Each leaf is 

 quadrifid, and examination of the base of the leaflets reveals 

 large reddish swellings or pulvini. Mechanical stimulus is 

 followed by a latent period of 3| seconds ; then slow but 

 accelerating contraction is observed for the next 4 seconds, 

 when rapid contraction follows for 22-23 seconds, and a very 

 gradual slowing down goes on till contraction ceases 80-90 

 seconds after stimulation. The striking and main difference 

 between this species and Oxalis stricta consists in the con- 

 traction period being about twice as prolonged. As with 

 Oxalis stricta, summation series can be obtained that vary 

 with the. time-intervals between stimuli and the environmental 

 surroundings. 



Observation of the contraction and expansion movements of 

 this and other sensitive plants shows that each is made up of 

 numerous minor contraction and expansion waves that cause 

 the leaf to move by minute jerks, each minor contraction phase 

 during the great period of contraction being much greater in 

 amount than the succeeding expansion phase. In all prob- 



