55° 



NA TURE 



[October 21, 1922 



pinnules and the bending of the petioles of Mimosa 

 could traverse portions of the petioles or stems which 

 had been raised to such a temperature as would kill 

 the living elements in these organs. Notwithstanding 

 that observation, Haberlandt's view, that the stimulus 

 is transmitted as a wave of pressure through certain 

 tubular elements of the bast, was generally accepted 

 as the least objectionable of any of the theories which 

 had been put forward to explain this transmission. 

 Ricca saw that, among other difficulties, the slowness 

 of transmission — never more than 15 mm. per second 

 — was a grave objection to this view. Accordingly, 

 working with a woody species of Mimosa — Mimosa 

 Spegazzinii — he removed the whole bast and outer 

 tissues of the stem for as many as twenty-three centi- 

 metres 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 considerable length caused Ricca 

 to recognise that the stimulus is transmitted in the 

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

 Thus he was led to assign the transmission to the 

 transpiration-current. 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 transpiration 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 stimula- 

 tion. He was able to show further 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 trans- 

 piration-stream is the Iong-sought-for mechanism by 

 which the stimulus is transmitted throughout Mimosa. 



As the stimulus travels both in a basipetal and 

 acropetal direction we may assume that movement 

 of the transpiration-stream in a downward direction 

 is of normal occurrence in plants. 



Contemporaneous with, and subsequent to, Ricca's 

 important work on Mimosa, experimental evidence 

 has been accumulating to indicate that the trans- 

 mission of other stimuli — phototropic, trauma- 

 totropic, 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 transmitted across protoplasmic discontinuities. 

 Paal emphasised this by showing that these stimuli 

 are able to pass a disc of the tissue of Anindo donax 

 impregnated with gelatine, which is interposed between 

 the receptive and responding regions. These observa- 

 tions 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 Ricca 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. 



NO. 2764, VoL. I 10] 



In all these cases it seems certain that the perceptive 

 tissues are the point of origin, when stimulated, of a 

 dissolved substance, the hormone, which makes its 

 way to the motile tissues and releases the response. 



In the case of Mimosa just alluded to, and of the 

 labellum of Masdevallia 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 

 trachea? 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 process. Some recorded velocities of transmission 

 are here enumerated for the sake of comparison : 



There is thus every reason to believe that the trans- 

 mission of stimuli generally through the tissues of the 

 higher plants is effected by the conveyance of a hormone 

 in the wood of the vascular bundles from the receptive 

 to the motile regions, and whenever this transmission 

 is in a downward direction evidence is afforded of the 

 downward movement of water in the trachea?. It is 

 reasonable to suppose that this downward current 

 is able to carry organic foodstuffs as well as hormones. 



Thus the evidence for the existence of a backward 

 flow of water in the trachea? of wood, in addition to the 

 more obvious upward stream, is convincing. With 

 regard, however, to the mechanism by which the back- 

 ward stream is supplied we have but scant information. 



The volume-changes of leaves which Thoday has 

 recorded are suggestive in this connexion. These 

 changes he found of various magnitudes, occurring 

 simultaneously in different or in the same leaves. 

 They may cause a linear contraction amounting to 

 2-5 per cent, in ten minutes, and may produce a volume 

 contraction of 7 per cent, in the same time. The water 

 corresponding to this volume-change in the cells of the 

 leaf if transmitted into the trachea? would produce a 

 considerable downward displacement, as may be seen 

 from the following figures : 



If these changes in volume are caused by, or accom- 

 panied with, a development of permeability of the 

 contracting cells, evidently a backward movement of 

 organic substance having a velocity of about 120 cm. 

 and more per hour would be produced. 



