DISCOVERY OF PROTOPLASM 107 



accumulating tending to show that these movements 

 were not purely physical or mechanical, but were of the 

 nature of responses to external stimuli, and that the 

 precise character of the response depended on the plant 

 organ and not on the stimulus. What then was it that 

 was sensitive or, in Knight's phraseology, what was " the 

 vehicle of irritation " ? 



Hooke, you will remember, was the first, in 1665, 

 to employ the term "cell," but he applied it to the 

 minute cavities he discovered in vegetable tissues, while 

 the walls were regarded by him and others after him as 

 composed of an " interstitial substance/ 1 As the micro- 

 scope improved, the attention of the early histologists 

 became more and more directed to discovering the nature 

 of these " interstitia," until Moldenhawer, by his macera- 

 tion method, finally demonstrated that each cell and fibre 

 had a wall of its own. Then arose a long controversy 

 on the nature, origin, and mode of thickening of the wall, 

 a subject we shall have to discuss presently. The cell 

 wall now came to be looked upon as the " cell " instead 

 of only something separating two or more cells from 

 each other. Everything inside the cell wall was lumped 

 together as " cell contents," " nutrient sap," " vital 

 juice," and so on. Presently, in 1831, Robert Brown 

 demonstrated the presence of a nucleus at least in many 

 cells, and, in 1838, Schleiden described the formation of 

 cells in the embryo-sac of the Phanerogam as a sort of 

 precipitation of " gum " round definite centres. This 

 " gum " was next shown by Naegeli to be nitrogenous, 

 and hence wrongly named from the chemical stand- 

 point. 



Meanwhile in the animal world, where the cell wall 

 was not nearly so prominent a feature, it had been 

 recognised that the substance of the cell was in the main 

 also nitrogenous, and to this substance Dujardin gave 

 the name of " sarcode." Since the sarcode, according 

 to the zoologists, appeared to be the active member and 



