I IS PALMER : 



flat surfaces which has been in contact with the slide, crawl 

 on and around the obstruction in that position, and come 

 down again beyond, flat once more upon the slide. It then often 

 proceeds persistently away in much the same direction it held 

 before all this trouble with the sand-grain.* These evolutions 

 remind one most forcibly of the tactics of Eunotia major 

 under similar conditions; and it is further to be added, that 

 like the Iiuuoiia, A^. soda/is is capable of crawling up the ver- 

 tical walls of a bottle, and may even move for a time on the 

 under side of a cover glass. In this last position, however, 

 it would not appear as securely placed as some other diatoms, 

 and Navicula Iridis frequently pushes it loose, so that it sinks 

 ilown through the water to the slide. 



Having now, as I believe, shown the permanent and 

 essential nature of the grouping during the whole of the veg- 

 etative period, I desire to suggest a working hypothesis as to 

 the origin of these groups, unique as they appear to be 

 among diatoms. 



It is well known that many, if not most, diatoms cast olT 

 their silicious parts on reaching a certain minimum length 

 after successive reduplications, and by one or another process 

 resume their original size. Whether with or without conju- 

 gation with other individuals, spore-like bodies are formed, 

 which after varying transformations in different genera pro- 

 duce one or two new, large diatoms that (juickly secrete for 

 themselves new valves according to the pattern proper to the 

 particular species. Several types of this rejuvenescence have 



*I have not given here a description of an isolated occurrence. The 

 above, or its eqiii\alent, has heen oliserved a nnniher of times. I'or the 

 benefit of those numerous writers who prefer to have all the motile phe- 

 nomena reduced to terms of "reactions" and "tropisnis," I would 

 suggest, with the diflTidence proper to one unskilled in such matters, 

 that N. socialis reacts positively to the near side, and negatively to the 

 far side of the grain of sand. In this connection I cannot easily forbear 

 the further remark, that we are some distance away, in this veracious 

 account, from motile theories of inward or outward streams of gas, 

 water or "gallerte ! " 



