ZYGOSPORES. 261 



old valve of the mother-cell; this latter extends, like the lid of a box, over the newly- 

 formed valve; and the two valves of the two daughter-cells lie next one another. 

 Since, according to Pfitzer, the silicious valves, which also contain some organic matter, 

 do not grow, it is clear that the new cells must constantly become smaller from 

 generation to generation. When they have thus attained a certain minimum size, 

 large cells, the Auxospores, are suddenly formed ; the contents of the small cells, 

 leaving the silicious valves which fall apart, increase either simply by growth or by 

 both growth and conjugation. After this the auxospores surround themselves with 

 new valves. Since the large auxospores are of somewhat different shape to their 

 smaller mother-cells and primary mother-cells, the first result of their division must 

 necessarily be cells of a different form and with unequal halves, as in the Desmidieae. 

 The formation of the auxospores has been more exactly followed out only in a few 

 cases. It would appear that they are formed in very different ways, from two or from 

 one mother-cell, singly or in pairs, and with or without conjugation ; they are alike 

 only in so far that their size greatly exceeds that of the mother-cell. It has already 

 been mentioned that, according to the recent researches of Schmitz in the case of 

 Cocconema Cistula, the auxospores are produced by two cells laying themselves parallel 

 side by side and allowing their contents to escape by the opening of the two valves ; the 

 cell-contents then commence growing rapidly without coming actually into contact, 

 and a pair of auxospores is thus formed. Diatoms are found in enormous numbers at 

 the bottom both of the sea and of fresh water, and attached to the submerged parts of 

 other plants. Besides the ordinary rotation of protoplasm in their interior, they also 

 exhibit a creeping motion by means of which they crawl over hard bodies or push small 

 granules along their surface. This occurs only in a line drawn along the length of 

 the cell- wall, in which Schultze^ supposes crevices or holes through which the pro- 

 toplasm protrudes; and this, although not yet actually seen, probably occasions the 

 creeping motion. 



FORMS NOT CONTAINING CHLOROPHYLL. 



A. Conjugation takes place between motile cells (myxoamoebse). 



The Myxomycetes^ differ so greatly in external appearance from the rest of the 

 Thallophytes that they have been by some altogether separated by the vegetable 

 kingdom. But if attention be directed not so much to the peculiarities of their external 

 appearance as to those points in each stage of their development w hich are important from 

 a morphological point of view, and especially to the fact that the formation of a cell-wall 

 is a point only of secondary importance in the lower Thallophytes generally, it becomes 

 clear that the Myxomycetes are allied in all essential points to the Pandorineae ; and 

 even if their mode of life is so remarkably different from that of this group of plants, the 

 chief cause of this is that they do not, like them, live in water, but in the interstices of 

 moist substrata, in consequence of which their motion is necessarily of a very different 

 kind from that of the Pandorineae. In place of swimming and rotating, we have in the 

 Myxomycetes a creeping of amoeboid masses of protoplasm such as alone is practicable 

 on the substratum on which they grow. It is impossible, in the space we have here at 

 command, to go into these relationships more in detail ; it must suffice to trace the 

 main points in the history of their development, and to point out, in passing, the most 

 important analogies. 



' [See Pop. Sci. Rev. 1866, p. 395— Engelmann, Bot. Zeit. 1S79, p. 49.] 



^ De Bary, Die Mycetozoen, 1864. [Ann. Nat. Hist, i860, vol. V. p. 233.] — Cienkowski in 

 Jahrb. fiir wiss. Bot. vol. III. pp. 325, 400. — Brefeld, Ueber Dictyoslelium mucoroides, 1869, Abhandl. 

 der Senkenberg. Gesellsch., vol. VII.— Rostafinsky, Versuch eines Systems der Mycetozoen, 1873.— 

 Brefeld, Ueber Mucor racemostis uiid Hefe, Flora 1873 (latter part of the article). 



