106 HISTORY OF BOTANY 



the middle of the century that Cryptogams received that 

 attention to which they were entitled. For the present 

 therefore we may leave the matter on one side and turn 

 to the consideration of two subjects of fundamental 

 importance in the history of Biology, viz. the discovery 

 of protoplasm and the foundation of the cell theory. 



PROTOPLASM AND THE CELL THEORY 



Let me recall to your memory the condition in which 

 we left the conception of plant movement. Knight, in 

 1812, after examining with wonderful skill the mysterious 

 performances of roots and stems in relation to gravity, 

 and the equally puzzling contortions of vine leaves in their 

 efforts to regain their original orientation with reference 

 to incident light, finished up with the confession that he 

 could not explain the movements " without admitting 

 not only that the plant possesses an intrinsic power of 

 moving but that it also possesses some vehicle of 

 irritation." 



In the years following the publication of Knight's 

 classic researches numerous additional instances of 

 movement were recorded, and also many new observations 

 on such movements as had already been recognised as 

 taking place under certain conditions. That roots when 

 placed in a horizontal position did not bend downwards 

 simply on account of their own weight became evident 

 after it had been demonstrated that they could push 

 down the scale-pan of a balance against the resistance of 

 a weight in the other pan, and could even force their 

 apices into mercury. Dutrochet and Von Mohl went 

 the length of ascribing to tendrils, twining stems, and 

 mobile leaves a certain degree of sensitiveness, and 

 regarded the contact with the foreign body as the stimulus. 

 Dutrochet also found that some stems, such as those of 

 parasites, were negatively heliotropic while others were 

 positively geotropic. In short, evidence was rapidly 



