152 ANIMALS AND PLANTS. [LEOT. 



most rapid revolution which biological science has 

 ever undergone was effected by the application of the 

 modern microscope to the investigation of organic 

 structure ; by the introduction of exact and easily 

 manageable methods of conducting the chemical 

 analysis of organic compounds ; and finally, by the 

 employment of instruments of precision for the mea- 

 surement of the physical forces which are at work in 

 the living economy. 



That the semi-fluid contents (which we now term 

 protoplasm) of the cells of certain plants, such as the 

 Cliarce, are in constant and regular motion, was made 

 out by Bonaventura Corti a century ago ; but the fact, 

 important as it was, fell into oblivion, and had to be 

 rediscovered by Treviranus in 1807. Kobert Brown 

 noted the more complex motions of the protoplasm in 

 the cells of Tradescantia in 1831 ; and now such 

 movements of the living substance of plants are well 

 known to be some of the most widely-prevalent pheno- 

 mena of vegetable life. 



Agardh, and other of the botanists of Cuvier's 

 generation, who occupied themselves with the lower 

 plants, had observed that, under particular circum- 

 stances, the contents of the cells of certain water-weeds 

 were set free, and moved about with considerable 

 velocity, and with all the appearances of spontaneity, 

 as locomotive bodies, which, from their similarity to 

 animals of simple organisation, were called " zoo- 

 spores." Even as late as 1845, however, a botanist of 

 Schleiden's eminence dealt very sceptically with these 

 statements ; and his scepticism was the more justified, 



