250 PLANT PHYSIOLOGY 



of the pulvini.* These cells, taking up water from the 

 adjacent vascular elements in the axial strand of bun- 

 dles, expand, become turgid, and exercise sufficient force 

 to raise the petiole or the blade respectively of the leaf 

 or leaflet above. The opposite effect follows, leaflet, leaf, 

 and petiole droop when, for any reason, the parenchyma 

 cells of the pulviai, giving up the water which they con- 

 tain to the vascular elements, become smaller, flabby, and 

 contracted, if not coUapsed. 



A great variety of stimuli set in motion the mechanism 

 by which the leaflets and leaves are closed. What is the use 

 of these movements, and what is the means of transmit- 

 ting the impulse to move, are by no means clear. Stahl,t 

 from observations made in the tropics, confirms Darwin's 

 claim that the closing of the leaves is an effective protec- 

 tion, in connection with the thorns which the plant bears, 

 against hungry herbivorous mammals. It is seemingly 

 probable that contact and various other stimuli cause the 

 contraction of some of the cells in the leaf and that, in their 

 contraction, these cells expel water into the Vascular ele- 

 ments or intercellular spaces. The contraction of any cell or 

 group of cells will necessarily affect the tensions of adjacent 

 cells. Thus, though we cannot conceive of the transmission 

 of the impulse itself from cell to cell, yet we can readily 

 conceive of the extension to an increasing number of cells of 

 the conditions first produced by the irritant acting upon a 

 small number of cells. Mimosa is not able to respond to 

 stimulus (whether it is then sensitive or not is another 

 question ) when under the influence of an anaesthetic or in a 

 state of chiU, heat-rigor, etc. For this reason the inference 

 is easy that the physical changes induced by stimuli are 



* Stahl, E. ilber den Pflanzenschlaf und verwandte Erscheinungen. 

 Bot. Zeitung, 1897. Cunningham, D. D. The causes of the fluctuations 

 in the motor organs of leaves. Annals of the Botanic Garden, Calcutta, 

 vol. VI., 1895. MacDougal, D. T. The mechani m of movement and 

 transmission of impulses in Mimosa and other " Sensitive" Plants : a 

 review with some additional experiments. Bot. Gazette, XXII., 1896. 

 Haberlandt, G. Das reizleitende Gewebe der Sinnpflanze, Leipzig, 1890. 

 Sinnesorgane im Pflanzenreieh. Leipzig, 1901. 



t Stahl, E. 1. c. 



