558 THE NATURAL HISTORY OF 



and directed to some object ; whilst that of plants would 

 appear to depend upon external physical causes or stimuli ; 

 that it is not voluntary, nor directed to any object, 

 but automatic (vide Ehr., Abh. d. Berl. Ak. 1830). 

 Cohn, however, from numerous observations expressly 

 directed to this point, is disposed to call in question, the 

 existence of this essential distinction between the motions 

 of the Infusoria, and that of the vegetable zoospore. 



Leaving out of the question the more highly organised 

 Infusoria furnished with the manifest mouth and oeso- 

 phagus, the motion of a large part of the Anentera, Ehr. 

 Astoma, Siebold, is not essentially different from that 

 of the zoospores of certain Algae. 



Towards the end of the paper, Cohn observes, that in 

 the course of its preparation he had only been able upon 

 optical and physiological grounds, to render it probable, 

 that the rigid cell membrane of the still form and the 

 tender enveloping cell of the motile zoospores, consist 

 of the same non-nitrogenous material, of which the 

 rigid membrane of all plant-cells is composed viz. : 

 cellulose ; he had, however, since succeeded, by chemical 

 means, in placing this fact beyond doubt. If a drop of 

 water containing some still and motile zoospores, be 

 brought in contact simultaneously with a very dilute 

 watery solution of iodine, and moderately diluted sul- 

 phuric acid, the " enveloping" cells of the motile and 

 the cell-membrane of the still form, immediately assume 

 a beautiful blue colour (Fig. 47). In performing this 

 experiment, it is necessary to employ neither a too con- 

 centrated, nor a too much diluted sulphuric acid. 



The author concludes by some general observations on 

 the subject of the "Alternation of Generations," exhibited 

 in such instances as Protococcus pluvialis, and on the im- 

 portance of the history of its development, with relation to 

 a systematic arrangement of the AlgaB. Most of our spe- 

 cies and genera, are based merely upon differences in size, 

 form, and thickness of the cell-wall, and the colour, con- 

 sistence, and intimate organisation of the contents. But 



