422 



FROTOPHYTA 



the genus Navicula, possess the power of propcHing themselves through 

 the water with considerable rapidity backwards and forwards in the 

 direction of their longer axis, often with a jerking motion, or of creeping 

 along the bottom on some submerged substance. The cause of this 

 motion is a subject on which a large amount of attention has been be- 

 stowed. Xiigeli attributed it to osmotic currents passing through the 

 cell-wall. Ehrenberg believed that he had actually seen, in some cases, 

 the extrusion through the raphe of vibratile cilia, in other cases of a 



' foot ' or pseudopode ; but 

 ^•/^ lf\ ^^^ observations have not 



been confirmed by others. 

 The explanation of the mo- 

 tion now generally accepted 

 is that of Schultze — ^that it 

 is due to the contractility 

 of the protoplasm which is 

 exuded outside the cell-wall. 

 Mereschkowsky (Bot. Zeit., 

 1880, p. 529) states the 

 arguments in support of the 

 various views with regard 

 to the causes of the mo- 

 tion, and sums up in favour 

 of the theory that it is the 

 result of osmotic currents 

 within the siliceous cell-wall. 

 Hallier, again (Unters. liber 

 Diatomeen, 1880), considers 

 it due to a contractile layer 

 of protoplasm, and asserts 

 that at an early stage di- 

 atoms have no true cell- 

 wall of cellulose. Onderdonk 

 (Microscope, 1885, p. 205) also attributes it to 'external cyclosis.' 



Diatoms have three modes of multiplication : — by simple division, 

 by auxospores, and by a kind of conjugation which is regarded by some 

 as sexual ; but the three modes pass gradually one into another. Simple 

 division always commences with the bipartition of the nucleus. When 

 it is about to commence the two valves separate from one another, the 

 contents divide into two daughter-cells, and new siliceous valves are 

 formed inside the old ones, and therefore necessarily smaller than they. 

 The valves of the new individual are formed necessarily one after the 

 other, the one formed later being smaller. The individuals produced in 



Fig. 355.— Stages in the formation of the auxospore of 

 Friistulia saxonica Ag. s, valves ; in, gelatinous en- 

 velope ; c, endochrome-plates ; a, auxospore (x 1,200). 

 (After Pfitzer.) 



