OF THE DIATOME.E. 



'9 



to each other or to delicate Confei-vae, appear branched. In the young con- 

 dition, when examined under shallow magnifiers, they resemble gelatine ; but 

 with a power of 300 diameters the flexible granules are discoverable, and, 

 with dextrous management, the little chains forming the felt or fioceose 

 web can be made out. In summer, on the other hand, its structiu^e can be 

 observed much more easily and distinctly. Early in spring the coloiu' is that 

 of a pale yellow ochre, but in summer that of an intense rusty red. Other 

 examples occur where a single species becomes tangible to the unaided senses ; 

 such are met with in the brown specks mentioned in the preceding account of 

 habitats formed by particular species upon the larger Algse and Confervse. So 

 the GompJionema geminatum forms on rocks tufts of a spongy texture and 

 brownish coloiu' when young, but white aftei^wards. The St/nedra Uhm often 

 produces a white incrustation on stones in rivers in summer ; and Fragilaria 

 and Odontidium are seen outstretched as dehcate brown filaments, several 

 feet in length, like many filiform Algce, from which, however, they differ by 

 breaking up so very readily, on the least disturbing force, into their separate 

 joints. ^' Large numbers of Rhizoselenia'' (writes Mr. Brightwell, J. M.S. 

 1858, p. 95) "have been detected in the stomachs of Salj^ce, and they have also 

 been observed floating free in the ocean in vrarm latitudes, their appearance 

 being that of httle confervoid flakes of exquisite dehcacy, but of a sufiicient 

 aggregation of filaments to be seen by the naked eye. The mass appeared 

 (probably from the endochrome) of a faint, evanescent, ochraceous colour." 

 Moreover, the frondose species generally attain an appreciable magTiitude. 

 Thus Encyonema prostratum forms a tuft-like stratum, — when recent, dark 

 brown, but when dried, of a dull green colour. Schlzonema suhcohcerens grows 

 into tufts from a quarter to half an inch or more high ; and S. vidgare con- 

 stitutes a dark bro^Ti gelatinous stratiun on stones in shallow water, fila- 

 ments simple or nearly so in deep still water, and much branched filaments 

 in deep rapid streams. 



Mr. I^orman, of Hull, has most kindly furnished us with the following 

 original observation on the growth of one species, the Campylodiscus cos- 

 tatus : — " In the early part of the spring of 1856," he "«T:ites, " I made a 

 gathering of freshwater Diatomere from the ' Spring Ditch,' Hull. Although 

 I met Tvith a few odd finistules of the species named, I did not consider it of 

 sufiicient interest to boil in acid for moimting, and the phial containing them 

 was left in the window of my laboratory during the ensuing summer. Some 

 time in the autumn I had occasion to make use of this bottle, and was on the 

 point of thro^ving away the contents, when I noticed the sui'face of the de- 

 posit and the sides of the bottle to be covered A\'ith a dense brown growth of 

 Diatoms. On further exarmination I found an immense colony of Campylo- 

 discus, which gave by preparation some beautifully pui'e slides of thi§ species. 

 In removing the upper layer I purposely left a few of the frustules in the 

 bottle, which was again placed in the window. These have again increased 

 to a great extent, and now (December 1857) they appear to thrive in perfect 

 health. Does not this occiUTence suggest an easy plan of prociu^ing in a 

 pure state such forms as are rarely found together in any abundance ? " 



Geogeaphical Distribution. — Species of Diatomeae are for the most part 

 distributed over a very wide geographical area. Some, indeed, would seem 

 cosmopolitan, whilst others are limited to certain regions. For instance, the 

 Terpsinoe has not been discovered in Eiu'ope ; and Synedra Entomon is 

 reckoned by Ehrenberg as peculiarly a South American production. This 

 author has given full force to this seeming fact, and employed it in the en- 

 deavour to discover the origin and course of meteoric dust, and also to arrive 

 at certain geological deductions. For example, he says {Monatsh. Berlin, 



