ANIMALS AND PLANTS. 159 



revolution which biological science has ever undergone 

 was effected by the application of the modern micro- 

 scope to the investigation of organic structure ; by the 

 introduction of exact and easily manageable methods 

 of conducting the chemical analysis of organic com- 

 pounds ; and finally, by the employment of instruments 

 of precision for the measurement 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 

 Charce, 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 re- 

 discovered by Treviranus in 1807. Robert 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 phenomena of vegetable life. 



Agardh, and other of the botanists of Cuvier's gen- 

 eration, who occupied themselves with the lower plants, 

 had observed that, under particular circumstances, 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 sim- 

 ple organisation, were called " zoospores." Even as late 

 as 1845, however, a botanist of Schleiden's eminence 

 dealt veiy sceptically with these statements; and his 

 scepticism was the more justified, since Ehrcnberg, in 



