IRRITABILITY 



249 



The plants of Mimosa pudica shown in the accompanying 

 figures (21) were stimulated not by touch but by a small 

 flame, though a touch or blow of sufHc- 

 ient force would produce the same ef- 

 fect. The figures therefore illustrate 

 what there is to say of the relations of 

 this plant to contact. Comparing 

 these with figure 20 a on p. 248 we find 

 that under favorable conditions of 

 light, temperature, moisture, etc., 

 lightly touching one or more of the leaf- 

 lets of a plant with the finger, a pencil 

 or some similar harmless instru- 

 ment, will induce the leaflets to move 

 quickly upward toward one another 

 and slightly forward, so that they 

 come to lie closely face to face along 

 the leaf-stalk. An exceedingly light 

 touch may induce only one leaflecto 

 move, a touch less light will induce 

 both leaflets of a pair to move, one 

 still stronger wUl stimulate adjacent 

 pairs and finally all the leaflets of 

 the compound leaf to close. At last 

 the main petiole of the whole leaf 

 sinks, bending at its pulvinus. as is 

 shown in figure 21 b. If the touch 

 be sufficiently strong, a blow rather 

 than a touch, the other leaves and 

 leaflets will behave in all respects 

 similarly (figure c) until finally the 

 appearance of the plant will be such 

 as indicated by figure d. 



Although evidently there is still 

 much work to be done before the 

 subject will be quite clear, it appears 

 from already published investiga- 

 tions that the means by which ^. „_„.,. ™ 



, , 1 1 n i T Figure 21. Sensitive Plant in 



the leaves and leaflets are moved various stages of stimulation. 

 are found in the parenchyma cells From MacDougal. 



