6l6 MICKOSCOPIC FOKMS OF VEGETABLE LIFE- THALLOPH YTKS 



by a longitudinal line which sometimes has a nodule at each end 

 (as in Navicula), the lower valve (a) has also a transverse line form- 

 ing a stauros, or cross, which is wanting in the upper valve (e). A 

 persistence of the connecting membrane, so as to form an additional 

 connection between the cells, may sometimes be observed in this 

 genus ; thus in fig. 461 it not only holds together the two new 

 frustules resulting from the subdivision of the lowest cell, a, which 

 are not yet completely separated the one from the other, but it may 

 be observed to invest the two frustules b and c, which have not 

 merely separated, but are themselves beginning to undergo binary 

 subdivision ; and it may also be perceived to invest the frustule d, 

 from which the frustule e, being the terminal one, has more com- 

 pletely freed itself. 



In the family Cymbellece, on the other hand, both valves possess 

 the longitudinal line with a nodule in the middle of its length ; but 

 the valves have the general form of those of the Eunotiece, and the 

 line is so much nearer one margin than the other that the nodule 



is sometimes rather mar- 



A B ginal than central, as we 



see in Cocconema (fig. 

 468,/). 



The Gomj}honemece, 

 like the Meridiece and 

 Liemophorece, have frus- 

 tules which are cuneate 

 or wedge-shaped in their 

 front view (figs. 462, 463), 

 but are distinguished 

 from those forms by the 

 presence of the longi- 

 tudinal line and central 

 JbiG. 463. Gomphonema qeminatum, more highly *, -, . -,,-, -, ,-, 



magnified : A, , side view of frustule ; B, front view ; nodule. Although there 

 C, frustule in the act of division. are some free forms in 



this family, the greater 



part of them, included in the genus Gomphonema, have their frus- 

 tules either affixed at their bases or attached to a stipe. This stipe 

 seems to be formed by an exudation from the frustule, which is 

 secreted only during the process of binary division ; hence, when 

 this process has been completed, the extension of the single filament 

 below the frustule ceases ; but when it recommences, a sort of joint 

 or articulation is formed, from which a new filament begins to sprout 

 for each of the half-frustules ; and when these separate, they cany 

 apart the peduncles which support them as far as their divergence 

 can take place. It is in this manner that the dichotomous character 

 is given to the entire stipe (fig. 462). The species of Gomphonema 

 are, with few exceptions, inhabitants of fresh water, and are among 

 the commonest forms of Diatomacece. 



Lastly, we come to the large family Naviculea, the members 

 of which are distinguished by the symmetry of their frustules, as 

 well in the lateral as in the front view, and by the presence of a 

 median longitudinal line and central nodule in both valves. In the 



