IRRITO-CONTRACTjLITY IN PLANTS. 203 



closely related species as Mimosa pudica and Mimosa lupulina, 

 the former has the most rapid motion in its leaf stalk at present 

 known, the latter seems to be quite stationary. 



It may now be asked, How do the seed leaves of sensitive 

 plants behave ? De Candolle first noticed that the cotyledons 

 of Mimosa are irritable to touch, while Bert, Pfeffer and 

 Darwin have confirmed his observations. To the last writer, 

 also, we owe a list of additional plants with sensitive cotyledons, 

 for he pointed out that several species of Oxalis, Mimosa, 

 Trifolium, Cassia and Lotus all show sleep movements, and 

 further that eight species of Cassia, a species of Smithia, 

 Mimosa pudica and Mimosa sensitiva, also Oxalis scnsitiva, 

 are irritable to contact. I have not only been able to add 

 several to Darwin's list, but have learned much as to their 

 extreme sensitiveness. Various of them are not merely irritable 

 to a single impact, but undergo a definite amount of contraction 

 that varies, as in the vegetative leaves, with age and environ- 

 ment. They contract to their fullest extent with a summation 

 of stimuli, they are highly irritable to heat and cold stimuli, 

 also to chemical agents that excite contractile tissue. 



Those of the sensitive plant are most active during the 

 period that their activity is of greatest benefit, viz, in the 

 very young state (seedlings 2-10 days old), since their great 

 function is to protect the first leaves and growing bud. When 

 the latter have pushed out above the cotyledonary tips, the pro- 

 tective function has ceased, and their irritable movements are 

 greatly lost. This, of course, is owing to the protoplasm 

 becoming senile, for as their irritability becomes less their 

 green color is transformed into yellow, and they then shrivel 

 and soon after drop off. This may have misled Darwin into 

 supposing that the seed leaves of Mimosa pudica were feebly 

 sensitive, for he speaks of their rising after irritation by 

 rubbing, or by tapping for from 30 seconds to 3 minutes, 

 but I cannot understand why he considered so much effort 

 necessary, unless it be that he happened to choose rather 

 old seedlings or experimented at low temperatures. 



The irritable movements of the cotyledons as compared with 

 the foliage leaves of Oxalis dendroides are of great importance. 



