PLANT SENSITIVITY 71 



movements of stamens, the " circulation " of cell contents 

 in some Thallophyta, the oscillation of the filaments of 

 certain Schizophyceae, and so on. All these mysterious 

 phenomena were explained either on purely physical 

 grounds or, especially towards the end of the century, by 

 dragging in a mysterious " vital force " which most 

 conveniently relieved the enquirer of the trouble of 

 making any further investigation. It was not indeed 

 until the early decades of the nineteenth century that 

 botanists began to feel dissatisfied with this explana- 

 tion which was in reality no explanation at all but 

 merely a confession of ignorance of the true cause of such 

 movements, viz. the sensitivity of living protoplasm to 

 external stimuli. You will be better able to appreciate 

 this change of outlook after we have considered the 

 experimental researches of Thomas Andrew Knight on 

 geotropism and heliotropism. 



In a previous lecture I asked you to notice how the 

 science of botany would be likely, as time went on, to 

 progress along certain fairly well defined lines. As you 

 doubtless remember, starting from the purely utilitarian 

 cataloguing and describing of the earlier herbalists, 

 there appeared, first of all a more or less conscious effort 

 to group plants according to their natural affinities, to dis- 

 cover, in other words, a rational classification. The search 

 for the principles on which such a classification could 

 be based led as a matter of course to the investigation 

 of the different organs of plants with the view of discover- 

 ing which of them might form the most satisfactory and 

 reliable " marks " to follow in arranging and grouping 

 the members of the vegetable kingdom as a whole. The 

 independent study of morphology led men to probe more 

 deeply into the structure of plants, and hence arose the 

 subject of anatomy, which is really morphology extended 

 and intensified. 



It is not consonant with the scientific mind to investi- 

 gate the structure of a piece of mechanism and refrain 



