524 THE NATURAL HISTORY OF 



ever, contradictory to the doctrine of Alternation of Gene- 

 rations, which is extensively applicable among the organ- 

 isms in question. For it is only after a series of divisions 

 that the true reproductive formation of spores takes place. 

 And Closterium, or Eunptia, which, after numerous divisions, 

 produce true spores by conjugation, differ from Spirogyra 

 and TJlothrix solely in this, that in the latter the parent- 

 cell, which connects each pair of divisional individuals, 

 being converted into a cuticular substance, is persistent, 

 whilst in the former it is soon dissolved. Whence it 

 is also incorrect to say — 2. that unicellular Algae are 

 normally separate and without organic connection, be- 

 cause the containing and interstitial gelatinous substance 

 should not be regarded as such. At all events I have 

 been unable to perceive any essential distinction between 

 the enveloping substance in Fragillaria and Mougeotia, 

 genera which are equally readily separable into distinct 

 cells, and in Nostoc and Apiocystis. 



As far as this, therefore, I agree with the reviewer of 

 Nageli's observations, (in Mohl and Schleclitendal's 

 'Botan. Zeit.,' 1849, Nos. 41-45,) but when he goes on 

 to assert " that there are no such things at all as uni- 

 cellular Algae, and that each cell is only part of a system 

 of cells contained one within the other, (1. c, p. 801,) it 

 is impossible for me to coincide with him. 



There are, undoubtedly, unicellular Algae, that is to 

 say. Algae, the fluid contents of which, sometimes con- 

 taining already organised particles, are enclosed in a 

 single, semifluid, nitrogenous envelope, and this again in 

 a cell-merabrane, often consisting of several layers of 

 different kinds ; and which, moreover, possess the faculty 

 of dividing themselves into several secondary cells, for 

 the most part equivalent to the primary cell. To these 

 unicellular Alga belongs Protococcus pluvialis. 



That this is the case is most clearly seen in the still 

 form, which is most distinctly characterised by its cell 

 membrane, a more or less thick, though always colour- 

 less, envelope. In some cases it is gelatinous, and then 



