520 TRANSFORMATION OF ENERGY 



noticed that the contraction, after stimulation, may amount to lo, 20, or even 

 30 per cent, of the original length. The filament has a very simple anatomical 

 structure, consisting merely of a delicate vascular bundle surrounded by paren- 

 chyma, which latter is the only part that is sensitive. When stimulated, this 

 tissue becomes less turgid and water enters the intercellular spaces, as in 

 Mimosa, while the filament decreases markedly in volume. If water be pre- 

 viously injected into the intercellular spaces, the water exudes by the cut surface 

 when the filament is stimulated, although in ordinary cases the intercellular 

 spaces are sufficient to accommodate the liquid which has been extruded. In 

 the Cynareae it may be definitely shown that a diminution in the expansion of 

 the cell-wall is not concerned, since its elasticity is quite as great in the con- 

 tracted filament as in the extended one, a fact which may be shown by 

 previously subjecting it to the action of chloroform and so preventing stimu- 

 lation from shock. Further, no change takes place in the condition of the 

 membrane during the stimulation, for a weight capable of stretching a contracted 

 filament out to its original length is also sufficient to prevent any contraction on 

 stimulation (Pfeffer, 1873 a ; 1890, 326). On stimulation, therefore, there 

 must be a decrease in the pressure of the cell-contents, apparently a decrease 

 in osmotic pressure, which, according to Pfeffer, must amount to from i to 3 

 atmospheres. The plant therefore does not make use of the whole of the elastic 

 play of the membrane, for the filament, after becoming contracted on stimu- 

 lation, may be still further markedly shortened by plasmolysis, while, on the 

 other hand, the filament when extended may still be stretched by mechanical 

 means, within the limits of its elasticity. The cell-walls, when the tension is 

 completely relaxed by plasmolysis, do not exhibit a permanent elongation, 

 if they be extended 100 per cent, by mechanical means ; but in their exten- 

 sibility, the filaments of the Cynareae stand quite alone in the vegetable king- 

 dom (p. 420). 



Sensitive filaments occur in other sub-orders of the Compositae besides the 

 Cynareae (Hansgirg, 1890), and curvature movements of filaments in response 

 to a shock stimulus have been noted in Cactaceae, Cistaceae, Mesembryanthema- 

 ceae, Tihaceae, Portulacaceae, and Berberidaceae as well. The filaments in 

 these plants are sometimes sensitive on one side, sometimes on all sides, and 

 the curvatures they exhibit are sometimes inwards, sometimes outwards; 

 but in all of them the nature of the sensitivity and the method of movement 

 appears to resemble the phenomena as exhibited by Mimosa. Transmission 

 of stimulus has also been recognized in Sparmannia by Hansgirg (1890). In 

 most cases these movements have some obvious relation to the cross-pollination 

 of the flowers. 



In addition to single sensitive styles {Arctotis ; Minden, 1901) there are stig- 

 mas whose lobes approximate when stimulated by contact ; they occur especially 

 in the Scrophulariaceae, Acanthaceae, Pedalineae, Bignoniaceae, and Cappari- 

 daceae ; these movements have been as yet very little investigated from the 

 physiological point of view. We are at liberty to assume, however, that the 

 approximation in them also is the result of a decrease in osmotic pressure. 

 In certain cases a transmission of stimulus takes place from one stigmatic 

 lobe to another, but the conduction is obviously effected in a way altogether 

 different from that seen in Mimosa. Oliver (1897) supposes that the stimulus 

 is transmitted by intercellular protoplasmic communication, at all events the 

 transference of the stimulus is effected just as well after the vascular bundle 

 has been cut through. After a simple contact, the stigma remains closed only 

 for a short time, and after opening again it is ready to receive a new stimulus. 

 BuRCK has made the interesting observation that the stigma of Mimulus htteus 

 remains closed if the contact be caused by pollen-grains ; Torrenia fottrnieri 

 closes its stigma permanently if it comes in contact with pollen from the 

 larger stamens, although it opens again if pollen from the smaller stamens or 



