IRRITO-COXTRACTILITY IX PLANTS. 187 



least, we have evidence for believing that the effects of it are 

 not fully overcome till 7 or 7.30 a.m., in June or July, 



The movements now described are characteristic of a large 

 series of plants belonging to many natural orders, but the 

 group OxalidccB of the order GeraniacecB, and the order 

 Leg2iininos(Z probably include between them about three- 

 fourths of the entire number. 



You are doubtless all aware, however, that leaves or leaf- 

 parts may exhibit movements which serve a different purpose 

 in the economy of the plants that bear them. The leaves of 

 Diojicea and Droscra, as well as tendriliform leaves and leailets, 

 are examples in point ; and the question naturally presents itself, 

 — How are these movements effected .-' Before attempting an 

 answer, we propose to lay before you the results of observa- 

 tions and experiments which may aid in the solution of 

 the question. 



Naturally, in the vegetable as in the animal world, irrito- 

 contractility can only be started by stimuli of a mechanical, 

 chemical, thermal, luminous, or electrical nature. For several 

 reasons, I can only treat briefly here the effect of the first three 

 forms of energy, and I will select plants for illustration in the 

 order that seems most suited for elucidating the subject. 

 First, we may recapitulate regarding Dioncsa. As my published 

 researches show,^ a summation of two mechanical stimuli is 

 ordinarily necessary to start contraction. Further, these must 

 be applied with a time interval between of at least i second ; 

 for if two stimuli are given in rapid succession, both are propa- 

 gated through the protoplasm as one wave, so far, at least, as 

 motion of contraction is the visible outcome. But though one 

 stimulus or two rapidly applied stimuli are insufficient to start 

 contraction of the leaf-halves, we know that active molecular 

 changes are in process, for the leaf-halves exhibit delicate but 

 visible wave impulses passing along them from base to apex, 

 while Burdon Sanderson has demonstrated that active elec- 

 trical chano;es are oroing on. After a second mechanical 

 stimulus, or three, if two are rapidly applied, the leaf closes 

 partially, — /. r., the marginal bristles loosely interlock. If 



1 Bot. Cont. Univ. Penn., Vol. I, No. i, 1892. 



