THOMAS ANDREW KNIGHT, ESQ. 15 



seriously to investigate the phenomena connected with them. 

 His experiments on the descent of the radicle excited great 

 attention among scientific horticulturists, and have perhaps 

 been more generally known than any of his other researches*. 

 The machinery by which he subjected seeds to rotary motion 

 during the process of germination was constructed by his own 

 hands, with no other assistance than that of an old carpenter, 

 who was not remarkable for his intelligence. A representation 

 of this machinery is given with the paper describing the experi- 

 ment in the present work, and also in Sir H. Davy's Agricultural 

 Chemistry, in which Sir H. adopts Mr. Knight's hypothesis, 

 that plants probably owe the peculiar direction of their roots 

 and branches almost entirely to the force of gravitation. Mr. 

 Knight however, in a paper published a few years later, details 

 some experiments which show, that certain other natural 

 causes may occasionally so far act in opposition to gravitation, 

 as to divert the radicle, as well as the fibrous roots, from the 

 direction which gravitation would have impelled them to 

 follow ^. 



The experiments on the effects of rotary motion in counter- 

 acting the effects of gravitation were repeated by M. Dutrochet 

 and other foreign physiologists, with various modifications, but 

 always followed by the same results. On this subject a corres- 

 pondence commenced between M. Dutrochet and Mr. Knight, 

 which was continued during the remainder of his life. 



Among other facts established by Mr. Knight's experiments 

 is, that the ascending sap undergoes a change in its progress 

 through the leaves, somewhat analogous to that which takes 

 place in the blood of animals in its passage through the lungs ; 

 and that this elaborated sap afterwards descends through the 

 bark, depositing in its course an inner layer of bark, and a new 

 layer of wood, while the old external bark cracks and peels off 

 as the stem or branch of the tree increases its dimensions, by 

 the annual deposition of a layer of fresh wood. His views as to 

 the vessels through which the sap ascends to the leaf have not 



* See below, Paper No. VII. f See below, Paper No. XIII. 



