2OO BIOLOGICAL LECTURES. 



Sachs, Pfeffer and others have noticed that Mimosa plants 

 when left unwatered for a few days lose their power of con- 

 tractility. This is true of all irritable plants that I have 

 examined, and is capable of ready explanation. 



My plan till now has been to lead you up from one or 

 two well-known plants that exhibit a rather sluggish irrito- 

 contractility to the true sensitive plant that is unique in its 

 physiology. I propose now to pass down the scale again and 

 briefly pass in review some common field weeds, that may, like 

 Oxalis stricta, be found around us here. Had time permitted 

 I should 'have pointed out how the sensitive plant is related 

 to its near ally, Mimosa lupulina, and this again to Mimosa 

 sensitiva, both of which I have had the opportunity of studying 

 from the Washington Botanic Garden, through the kindness of 

 Mr. Oliver. But from the last an easy transition is established 

 with our Eastern American weed, Cassia nictitans, while C. 

 chamaecrista unites it again with C. marylandica, that shows 

 a very feeble though measurable response to mechanical, 

 chemical, and other stimuli. 



Cassia nictitans, the wild sensitive plant, is generally stated 

 in botanical manuals to be " somewhat sensitive." When a 

 delicate mechanical shock is given to the leaf, close observation 

 will show that the leaflets almost instantaneously change 

 position slightly, but succeeding to this is what I can only at 

 present, for want of better knowledge, designate as a latent 

 period of 5^ seconds. Thereafter the leaf stalk falls through 

 an angle of 15-23 and the leaflets simultaneously move 

 forward and rotate inward so that their outer edges become 

 uppermost as in the sensitive plant. This is accomplished in 

 85-86 seconds on the average, and by the end of that time the 

 leaflets are half-closed. But if a second shock be given after 

 the effects of the first have ceased, the leaf will now fall through 

 an angle of 8-9 and the leaflets will come together till they 

 nearly lie face to face as in Mimosa pudica. A third stimulus 

 may even give slightly added results. 



An interesting feature in this species is the propagation of 

 stimuli along the leaf though to a feebler degree than in 

 Mimosa. As many of you must have observed, a brownish- 



