DIATOMS 181 



larger new cell ("rejuvenescence"). They generate by 

 the escape and union of the protoplasms of two contigu- 

 ous cells whose half-cells have separated, resulting in the 

 formation of one or two new and usually much larger 

 cells. Small biciliate isogametes have been doubtfully 

 reported in some marine diatoms. 



260. There are two general kinds (orders) of Diatoms, 

 namely, the Round Diatoms {Ewpodiscales) with the cells 

 mostly round in end view, and the Flat Diatoms 

 [N aviculales) with the filaments flattened in end view. 



261. The Round Diatoms are mostly .^^^_^.,.^_^ 

 marine and fossil. The ends of the cells X. 1 , 1 , .L-L 

 are usually marked radially with lines or (^ 

 rows of dots, as in Melosira, Coscinodiscus, fiq. 73._a 

 Actinodiscus, etc. Some Round Diatoms Meiosl?!'""'"' 

 form long filaments (Melosira). 



262. The Flat Diatoms occur abundantly as fresh- 

 water, marine, and fossil plants. The ends of the cells 



(transection of the flat filament) are often 

 marked transversely or pinnately by dots or 

 lines. In many of our most common Flat 

 Diatoms (e.g. Naviculaceae) there is a me- 

 dian longitudinal slit ("raphe") in the end 

 Fiat°Diatwm Wall, which probably has to do with the mo- 

 Baciiiaria. ' ^jj-^y exhibited by these plants (Par.' 174). 



263. Origin of Zygophyceae. It may be assumed that 

 the plants of this phylum have been derived from other 

 filamentous plants, and that the adhesion of cell to cell, 

 and the consequent formation of a multicellular plant 

 body, had become a well established habit long before 

 the peculiarties arose which set them off as Zygophy- 

 ceae. We must search among the Confervoideae of the 

 preceding phylum for the ancestral types from which the 

 Conjugate Algae may have descended. Such plants as 



