K.— BOTANY. 



201 



There is thus every reason to behave that the transmission 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 tracheae. 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 bacliward flow of water in 

 the tracheae of wood, in addition to the more obvious upward stream, 

 is convincing. With regard, however, to the mechanism by which the 

 backward stream is supplied we have but scant information. 



The volume-changes of leaves which Thoday has recorded are 

 suggestive in this connection. 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 tracheae would produce a con- 

 siderable downward displacement, as may be seen from the following 

 figures : — 



If these changes in volume are caused by, or accompanied with, a 

 development of permeability of the contracting cells, evidently a back- 

 ward movement of organic substance having a velocity of about 120cm. 

 and more per hour would be produced. 



It is possible that the tension which causes these contractions of the 

 leaf-cells at the same time acts as a stimulus to increase the permea- 

 bility of the plasmatic membranes of the cells ; and so one might imagine 

 that the development of a certain tension would automatically release 

 organic substances from the cells and draw them through the tracheae 

 downwards. Direct experiment on this point presents difficulties, but 

 it may be worth recordmg that when the internal osmotic pressure o-f 



