IRRITO-COXTRACTILITY IX PLAXTS. 1 89 



The conditions thus revealed by DioiicEa were so arresting 

 as to cause me to inquire whether similar phenomena might 

 not be demonstrated in other sensitive plants. Botanists 

 have long been acquainted with certain species which from 

 their contractile movements were aptly designated " sensitive 

 plants." These included species of Oxalis, Avcrr/ioa, Jlliniosa, 

 Cassia, ScJirankia, etc., some of which were considered to be 

 highly irritable, others less so, and others, again, scarcely 

 deserving the name. The best account hitherto given of the 

 action of some of these when affected by mechanical stimuli 

 is to be found in Pfeffer's PJlanzoipJiysiologic (pp. 224-254), 

 where Mimosa pndica and Oxalis acctosclla are chiefly dealt 

 with. 



Now I hope not only to show you that various transition 

 types exist between such roughly classified groups as are 

 expressed in the descriptions "very sensitive," "slightly sensi- 

 tive," "scarcely sensitive," but that under giv^en environmental 

 conditions all exhibit a definite irrito-contractility, characterized 

 by varying degrees of latent period, of contraction period, of 

 expansion period, and of summation effects, or, to put it in a 

 broader sense, that tJie same pJienomcjia of irrito-contractility 

 are encoiintered i)i the vegetable as in the animal kingdom. 

 The optimum temperature for most of the plants now to be 

 mentioned is an average of 26° C, exposure for a short time or 

 for a prolonged period to low temperatures (8°-i5° C) causing 

 an extension of the latent period, slower rate of contraction, 

 and a reduced power of conducting stimuli. 



As being both a common plant and a central type in its 

 physiological behavior, we may now take the field and wayside 

 weed Oxalis stricta, the yellow sorrel. When plants are grown 

 in a rather shady situation and exposed to a temperature of 

 i8°-24° C, the leaves have a rich green color, and the three 

 leaflets tos-ether form a triradiate rosette. We need not now 

 refer to structural details further than to say that at the base 

 of each leaflet is a little cushion composed chiefly of small, 

 densely aggregated, and vacuolated cells forming a typical 

 sensitive pulvinus. After a sharp but delicate mechanical 

 stimulus applied with a pencil or other instrument to a terminal 



