1RRITO-CONTRACTILITY IN PLANTS. 2OI 



red knob-like gland is situated on the upper surface of the 

 petiole, about 3 m. m. below the insertion of the lowest 

 leaflets. If a minute drop of carbonate of ammonia be 

 cautiously placed on it, contraction of all the leaflets ensues, 

 proof this that a continuity of irritable tissue exists between 

 it and the leaflets that are inserted above. The structural 

 relations of this gland with the pulvini of the leaflets is inter- 

 esting, but cannot be dealt with now. Ice and cold water, 

 hot water and dry heat stimuli are all irritants to it, as are 

 chloroform and ether of 1 5 Jfc strength and upwards. These 

 can all act so continuously that they cause the leaf to fall 

 and the leaflets to rise to the extent that is seen in the 

 nyctitropic state. 



The hog pea-nut (Amphicarp&a monoica), as delicate and 

 graceful as it is abundant, is convenient for study alike in the 

 field and laboratory, but we can expeditiously treat it along 

 with its companions of our woods and thickets, the tick trefoils 

 or Desmodiums. My attention has been mainly confined to 

 three species of the genus, viz., Desmodinm cancscens, Desmo- 

 dium panicnlatnm, and Desmodinm rotnndifolinm. They show 

 a degree of sensitivity in the order that I have given them. 

 The latent period in Ampliicarpcza and Desmoditim canesccns 

 under ordinary conditions is 3--3j- seconds, but when plants 

 are grown in a green-house it is shortened to 2|--2j seconds. 

 In Desmodiiim paniculatnm the motion is so slow that I have 

 failed as yet to determine it exactly. 



The period of contraction is considerably longer than in any 

 yet described and is for Amphicarpcsa i 50- 160 seconds on clear 

 dry days, and 180-200 on close moist days; for Desmodinm 

 canescens and Desmodinm panicnlatnm from 120 140 seconds. 

 The amplitude of movement in Amphicarpaa is greater, however, 

 for equal stimuli than in the two last, thus while Amphicarpcea 

 after one stimulus falls in the forenoon of a dry day through as 

 much as 65-70, the others seldom fall through more than 

 48-50. But a result got with some plants of Amphicarpcea 

 on a close, warm, but dull day, is worth recording. Like most 

 leguminous species it raises its leaflets during the parathermo- 

 tropic period so as to point the tips at the sun. At 12.30 a 



