B 



6l6 MICROSCOPIC FORMS OF VEGETABLE LIFE- THALLOPHYTES 



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

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

 ing a sta/iros, 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, , 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 binarv 

 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, 011 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- 

 ginal than central, as we 

 see in (Jocconema (fio- 

 468, /). 



The Gomphonemecv, 

 like the Meridiem and 

 Licmpphorece, have frus- 

 tules which are cuneale 

 or wedge-shaped in their 

 front view (figs. 462, 463), 

 but are distinguished 

 from those forms by the 

 presence of the longi- 



tu(h ' nal h '^ d central 

 nodule. Although there 

 are some free forms ill 

 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 carry 

 apart the peduncles which support them as far as their divergence 

 -.in 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 Nai+iculew, the members 

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

 \\ell m the lateral as in the front view, and by the presence of a 

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



cjeminatnm, more highly 

 magnified: A, side view of frustule; B, front view; 

 C, frustule in the act of division. 



