626 



THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



exhibit a curious creeping movement, which is explained as being due to an 

 external sheath or to filaments of protoplasm ; the median line (or "raphe ") shown 

 by certain forms (e.g. Navicula, fig. 369 *) is interpreted as a narrow slit at which 

 this external protoplasm is extruded. Diatoms propagate by continuous longitudi- 

 nal division; the valves are slightly separated, and division takes place parallel to 

 the faces of the valves. Each daughter-cell thus possesses one of the valves of the 

 mother-cell, and they complete their integument by secreting another on the side 

 away from it. The new valve is always slightly smaller than the other one and 



Fig. 369.— Diatoms. 



* Several individuals of Synedra UZna attached to a cell of an aquatic plant. 2 a single individual of Synedra Ulna more 

 higlily magnified, 3 and * Navicuta Liber, seen from the side and from in front. * and * Similar views of Xavicula 

 tumida, ' Triceratium Favus. 8 CampylodUcits spiralis. 9 PUurosigvia angulatum, 10 and i^ Grammatophora ser- 

 pentina. ^- and is Two views of Gowphonema capitatum, n Gomphoiiema capitatuin on branched stalits which are 

 attached to some algal filament i* Diatom vulgare; the cells hang together into a zigzag band, is and i" Fragxllaria 

 viresMiis, showing an individual from two aspects and a row of six joined together into a ribbon, is Cocco7iema Ci^tala. 

 19 Two individuals of Cocconema Cistula inclosed in a mucilagiuous envelope preliminary to auxospore-fomiation. so xhe 

 two auxospores tiave elongated, and the old cases are seen to right and left; there is no conjugation in this instance. All 

 highly magnified. 



fits under its rim, so that the Diatoms arising in this way become smaller and 

 smaller till a certain minimum is reached. When this diminution has gone on for 

 a certain period an enlargement is attained by the formation of what are termed 

 auxospores. The contents of the cell gather themselves together, they become free 

 from the valves, enlarge, and then put on new valves. In other cases an actual 

 conjugation takes place, two individuals uniting into an auxospore; or each may 

 divide into two daughter-cells, which fuse in pairs, forming two auxospores. Fig. 

 369 '^ shows two cells of Cocconema Cistula embedded in mucilage previous to 

 auxospore-formation; in fig. 369-" each cell has escaped from its valves, and has 

 doubled its original length. In this instance, however, there is no accompanying 

 conjugation. 



Diatoms are very widely distributed over the globe, and occur in ([uantities in 



