94 MOVEMENTS OF CURVATURE 



per minute, for ten or twenty seconds after the head of a tentacle has been 

 strongly stimulated a curvature may be shown in a region of the stalk, 

 distant 2 or 3 mm. from the head. These times, it is true, include the 

 latent periods of perception and reaction, which can only be eliminated by 

 comparing the times at which curvature is shown at varying distances from 

 the head. In most plants, however, stimuli travel still more slowly than 

 in Drosera, and in this way plants largely avoid the waste of energy which 

 would be involved in a continual attempt to adjust themselves to transitory 

 variations in the external conditions. 



The exact determination of the velocity of propagation of stimuli 

 is difficult, and even when measured by the appearance of a reaction the 

 rate will depend not only upon the specific nature of the plant but also 

 upon the external conditions. A fall of temperature, a decrease of 

 turgidity, and the action of chloroform all lower the rate of transmission 

 of stimuli in Mimosa and in other plants, and ultimately produce a com- 

 plete cessation of conductivity l . A result of this kind may be due either 

 to a direct action upon the conductivity, or to an action upon the sensitivity, 

 excitability, or power of response, or may be due to a combination of these 

 factors. The importance of a close study of the influence of the external 

 conditions upon conductivity, excitability, and the power of response cannot 

 be overestimated, and the fact that stimuli may travel through etherized or 

 dead pieces of the stem of Mimosa shows that in this case the transmission 

 of seismonic stimuli is not vital in character. 



Dutrochet 2 was the first to show that stimuli are conducted through the vascular 

 bundles of Mimosa pudica, and he also came to the correct conclusion that the 

 transmission was due to a pulsation of water. Pfeffer 3 subsequently showed that 

 the stimulus was able to travel over chloroformed parts of the stem, and Haberlandt 

 found that dead regions of the stem and leaf retained their conductivity some time 

 after they had been killed*. We are, therefore, fully justified in ascribing the 

 transmission of the stimulus to the movements and changes of pressure of the water 

 in the vascular bundles, and when a cut is made in the stem, a stimulus is only 

 exercised when the knife penetrates the vascular bundles and allows the escape 

 of a drop of water. The stimulation of the neighbouring leaves at once follows, and, 

 as the stimulus spreads, all the leaves and leaflets may be in succession excited when 

 the plant is in a highly irritable condition. No stimulation or conduction takes 

 place, however, when the plant's turgidity is so low that no drop of water escapes 



1 Cf. Pfeffer, 1. c., p. 326. 



2 Dutrochet, Recherch. anat. et physiol., 1824, p. 69; Mem. p. servir a 1'histoire d. ve'ge'taux, 

 Bruxelles, 1837, p. 2 7 2 - 



3 Pfeffer, Jahrb. f. wiss. Bot., 1873-4, Bd. ix, p. 308. 



* G. Haberlandt (Das reizleitende Gewebesystem d. Sinnpflanze, 1890, p. 35) observed a pro- 

 pagation of the stimulus over a locm. length of dead stem ; Macdongal (Botanical Gazette, 1896, 

 Vol. xxn, p. 296) over as much as 30 cms. The mode of treatment and the maintenance of turgidity 

 are factors of considerable importance. 



