168 ON THE MOTIONS OF THE TENDRILS OF PLANTS. 



probably by ^occasioning the presence of a larger portion of the fluid 

 organisable matter of the plant upon the one side, than upon the other. 

 The external pressure of any body upon one side of a tendril will probably 

 drive this fluid from one side of the tendril, which will consequently 

 contract, to the opposite side, which will expand ; and the tendril will 

 thence be compelled to bend round a slender bar of wood or metal, just as 

 the stems of germinating seeds are made to bend upwards, and to raise 

 the cotyledons out of the ground ; and in support of this conclusion I 

 shall observe, that the sides of the tendrils, where in contact with the 

 substance they embraced, were compressed and flattened. 



The actions of the tendrils of the pea were so perfectly similar to those 

 of the vine, when they came into contact with any body, that I need not 

 trouble you with the observations I made upon that plant. An increased 

 extension of the cellular substance of the bark upon one side of the 

 tendrils, and a correspondent contraction upon the opposite side, occa- 

 sioned by the operation of light, or the partial pressure of a body in 

 contact, appeared in every case, which has come under my observation, 

 the obvious cause of the motions of tendrils ; and therefore, in conformity 

 with the conclusions I drew in my last memoir, respecting the growth of 

 roots, I shall venture to infer, that they are the result of pure necessity 

 only, uninfluenced by any degrees of sensation, or intellectual powers. 



XV. ON THE ACTION OF DETACHED LEAVES OF PLANTS. 



[Read before the ROYAL SOCIETY, June \3th, 1816.] 



SINCE I had last the honour to address a communication to the Royal 

 Society, I have repeated great part of the experiments which formed the 

 subjects of my former memoirs, with such additions and variations as 

 might probably lead to the detection of any erroneous conclusions which 

 I might have drawn ; but I have not been able to detect any errors, nor to 

 add anything very important to my former observations. I have, however, 

 been able to ascertain a few new facts, which I think too interesting to 

 be lost. 



I endeavoured, in my former communications, to adduce evidence that 

 the matter, which becomes vitally united to trees, previously passes 

 through their leaves ; and I shall now proceed to state some facts, which, 

 I trust, will prove that a fluid possessing the power which I have 

 attributed to the true sap actually descends through the leaf-stalks. 



