MICROSCOPIC FORMS OF VEGETABLE LIFE. 283 



of conjugation between the adjacent cells of the ordinary diameter; 

 taking place before the completion of their separation. He describes the 

 endochrome of particular f rustules, after separating as if for the forma- 

 tion of a pair of new cells, as moving back from the extremities towards 

 the centre, rapidly increasing in quantity, and aggregating into a zygo- 

 spore (Fig. 169, 2, a, b, c): around this a new envelope is developed, which 

 may or may not resemble that of the ordinary frustules, but which re- 

 mains in continuity with them; and this zygospore soon undergoes binary 

 subdivision (3, a, b, c), the cells of the new series thus developed pre- 

 senting the character of those of the original filament (1), but greatly ex- 

 ceeding them in size. From what has been already stated ( 278), it 

 seems probable that a gradual reversion to the smaller form takes place 

 in subsequent subdivisions; a further reduction being checked by a new 

 formation of zygospores. Whether this formation partakes of the charac- 

 ter of ( conjugation ' (as supposed by Mr. Thwaites) is still doubtful; some 

 later observers regarding ' auxospores ' as simply enlarged forms of single 

 cells. 



282. Most of the Diatoms which are not fixed by a stipes, possess 

 some power of spontaneous movement; and this is especially seen in 

 those whose frustules are of a long narrow form, such as that of the 

 NaviculcB generally. The motion is of a peculiar kind, being usually a 

 series of jerks, which carry forward the frustule in the direction of its 

 length, and then carry it back through nearly the same path. Some- 

 times, however, the motion is smooth and equable; and this is especially 

 the case with the curious Bacillaria paradoxa (Fig. 171), whose frus- 

 tules slide over each other in one direction until they are all but 

 detached, and then slide as far in the opposite direction, repeating this 

 alternate movement at very regular intervals. * In either case the motion 

 is obviously quite of a different nature from that of beings possessed of a 

 power of self-direction. "An obstacle in the path," says Prof. W. 

 -Smith, " is not avoided, but pushed aside; or, if it be sufficient to avert 

 the onward course of the frustule, the latter is detained for a time equal 

 to that which it would have occupied in its forward progression, and 

 then retires from the impediment as if it had accomplished its full 

 course." The character of the movement is obviously similar to that of 

 those motile forms of Protophyta which have been already described; 

 but it has not yet been definitely traced to any organ of impulsion; and 

 the cause of it is still obscure. By Prof. W. Smith it is referred to forces 

 operating within the frustule, and originating in the vital operations of 

 growth, etc., which may cause the surrounding fluid to be drawn in 

 through one set of apertures, and expelled through the other. 2 " If," as 

 he remarks, "the motion be produced by the exosmose taking place 

 alternatively at one and the other extremity, while endosmose is pro- 

 ceeding at the other, an alternating movement would be the result in 

 f rustules of a linear form; whilst in others of an elliptical or orbicular 



1 This curious phenomenon the Author has himself repeatedly had the oppor- 

 tunity of witnessing. 



2 It has been objected to this view, by the Authors of the " Micrographic 

 Dictionary," that, if such were the case, the like movements would be frequently 

 met with in other minute unicellular organisms. But there are no other such 

 organisms in which the cell is almost entirely inclosed in an impermeable enve- 

 lope, so that the imbibition and expulsion of fluid are limited to a small number 

 of definite points, instead of being allowed to take place equally (as in other uni- 

 cellular organisms) over the entire surface. See Mereschkowski in " Journ. Roy. 

 JNlicrosc. Soc.," Ser. 2, Vol. i. <1881), p. 102. 



