200 SECTIONAL ADDRESSES. 



tissues of the stem for many centimetres — viz., twenty -three— and was 

 able to show that the stimulus was still transmitted. Similarly he 

 found that the stimulus was transmitted through narrow strips of the 

 wood from which even the pith had been removed. These experiments 

 and others in which the transmitting organ had been killed for a con- 

 siderable length caused Eicca to recognise that the stimulus is trans- 

 mitted in the wood and not in the bast, as had been previously held. 

 Thus he was led to assign the transmission to the transpiration-curi-ent. 

 He was able to confirm this conjecture by showing that the transmission 

 to the various leaves of a plant is largely controlled by the rate of 

 the transpiratio-n from the individual leaves. Thus, other things being 

 equal, a rapidly transpiring leaf receives the stimulus sooner than a 

 sluggishly transpiring one equidistant from the point of stimulation. 

 He further was able to show that the stimulus may be transmitted 

 through a glass tube filled with water, just as it is transmitted through 

 a dead portion of the stem. Evidently a hormone set free into the 

 transpiration-stream is the long-sought-for mechanism by which the 

 stimulus is transmitted thi'oughout Mimosa. 



As the stimulus travels both in a basipetal and acropetal direction 

 we may assume that movement of the transpiration-stream in a down- 

 ward direction is of normal occurrence in plants. 



Contemporaneously with, and subsequently to, Eicca's important 

 work on Mimosa, experimental evidence has been accumulating to indi- 

 cate that the transmission of other stimuli — viz., phototropic, trauma- 

 toti-opic, thigmotropic, and geotropic — is effected by means of the passage 

 of a dissolved substance. Boysen- Jensen appears to have been the 

 first to announce that phototropic and geotropic stimuli may be trans- 

 mitted across protoplasmic discontinuities. Paal emphasised this by 

 showing that these stimuli are able to pass a disc of the tissue of 

 Arundo donax impregnated with gelatine, which is interposed between 

 the receptive and responding regions. These observations rendered the 

 view that the stimulus is transmitted in the form of a hormone extremely 

 probable; and later Stark showed that this hormone is thermostable, 

 just as Eicca had done in the case of the hormone of Mimosa. Another 

 very interesting point discovered by Stark — working with traumatic 

 stimuli — is that the hormones are to a certain extent specific. Thus 

 if the perceptive tip of a seedling is removed from one plant and affixed 

 in position on another, the certainty of the response depends on the 

 genetic affinity of the two plants. 



In all these cases it seems certain that the perceptive tissues are 

 the point of origin, when stimulated, of a dissolved substance, the hor- 

 mone, which makes its way to the motile tissues and releases the 

 response. 



In the case of Mimosa just alluded to-, and of the labedlum of Masde- 

 vallia examined by Oliver, there is direct evidence that the transmission 

 of the hormone is effected by the vascular bundles. In Mimosa the 

 channels are more precisely localised as being the trachese of the wood. 

 Furthermore, the rapidity of transmission renders it certain that simple! 

 diffusion through the tissues of the plant will not account for the pro-j 

 cess. Some recorded velocities of transmission are here enumeratec 

 for the sake of comparison: — 



