INTRODUCTION TO CRYPTOGAMIC BOTANY. 125 



stem is never developed, except where the plant has sprung 

 from a spore. The siliceous coats of the cells are not only 

 variously marked, but beneath these and the lining membrane 

 there are often little channels. Along the line of suture 

 in disciform or circular frustules, but more generally at the 

 extremity of the valves only, when oblong, linear, or elongated, 

 there exist perforations in the silex, which permit the sur- 

 rounding water to have access to the surface of the iatemal cell 

 membrane. Without some such provision, it is almost impossible 

 that there should be any interchange between the contents of 

 the cells and the surrounding fluid. Biatomaceos were long 



Fig. 34. 

 Epithemia gihba, Kutz. 

 From Mr. Thwaites, in Ann. of Nat. Hist., vol 20, tab. 22.* The two 

 original individuals liave each split up into two frustules. 



behoved to be animals, and this view has more especially been 

 maintained by Ehrenberg ; but the discovery of the coupling of 

 fronds by Mr. Thwaites, and the confirmation of the fact by 



* I have two or three mounted specimens of different species, and 

 from amongst them, E. gibha, from Mr. Thwaites, oiDiatomace^ in con- 

 jugation, confirmatory in every respect of his observations. 



