270 



HARDWICKE'S SCIENCE-GOSSIP. 



Viewed with a strong light and under a considerable 

 power (+ 1200) they appear like a string of vesicles 

 united like a necklace. This is the Leptothrix rigidula. 

 The living frustule is never incommoded in its 

 movements by them, and when (under the micro- 

 scope) it strikes an obstacle in the midst of the water, 

 we see these threads fold up from their base, but 

 they straighten and stiffen again as soon as the 

 obstacle is past. Boiling in water, and the- action of 

 nitric acid sets free these threads, which are not of a 

 silicious nature. In addition to this, potash distends 

 them and alcohol does not green them, which proves 

 the absence of diatomine. It is evidently this parasite 

 which Ehrenberg and, since then, other naturalists 

 have taken for motile cilia. What Kiitzing said seems 

 to indicate that he also thought that these appendices 

 formed a portion of the diatom. I have a prepara- 

 tion from water where this same Leptothrix adheres at 

 the same time to Synedra parvula and to the filamen- 

 tous algae {Zygonema) upon which their Synedra is itself 

 parasitic ; also another preparation where it adheres 

 at the same time both to Staurosira parasitica and to 

 the Xiizschia linearis which supports it, thus offering 

 the curious phenomenon of three parasites superim- 

 posed in a space of the five to six hundredth part of 

 a millimetre ! 



Their development. — Every diatom commences its 

 existence in water and in the midst of a slightly 

 coloured mucilage, which is translucid and often 

 feebly visible. Whether it begins life as a germ, a 

 spore, or from deduplication by fission, the first vital 

 state is always a gelatinous amorphous mass in the 

 middle of which the young frustules appear. The 

 frustules have not then their striations so plain as 

 when they are perfect and free. This is important to 

 note, and it has been the cause of many errors in the 

 determination of species, especially when the in- 

 tensity of the striations forms one of the specific 

 characters. 



Their- reproduction. — Once fixed in a situation 

 which is favourable to them, their development 

 and their multiplication proceed with astonishing 

 rapidity. Numerous observations have proved 

 that their reproduction takes place : 1st, by germs 

 (sporules) ; 2nd, by direct deduplication ; and, 3rd, 

 by reproductive sacs (spores) which result from this 

 deduplication. 



The sporules are so minute that they have escaped 

 up to the present the eye of observers aided by the 

 best immersion lens, such as those of Spencer, Ross, 

 Powell and Lealand, Zeiss, Hartnack, and Praz- 

 mowki, &c. Ehrenberg thought that they were able 

 to divide by fission in one hour, and that thus in four 

 days a diatom could produce one hundred and forty 

 billions. 



A diatom, indeed, does deduplicate itself in an 

 hour, but only when it has arrived at the necessary 

 degree of development for its deduplication to take 

 place, for the works of W. Smith, Thwait, of Bre- 



bisson, and my own observations have proved that it 

 takes on the average six to ten days, from the state 

 of a germ, for it to be able to reproduce. 



Their collection. — It is by the borders of ponds or 

 brooks where the water is slightly deep and very 

 clear that we most find these microscopic algae. 

 One can easily detect their presence by great glairy 

 patches, yellow, tawny, or brown, which they form 

 at the bottom of the water. Sometimes also they 

 constitute that organic scum, soft, brownish, or 

 golden, which floats on the surface of stagnant water. 

 They may be found also adhering in great abundance 

 to the surface of submerged aquatic plants. They 

 form that tawny mucilage, sometimes clear brown or 

 greenish, which covers submerged stones, piles of 

 dams, lake jetties, floating wood, &c. They abound 

 in damp rocks of the Alps and Jura, and where 

 there are permanent springs and cascades ; or even 

 where glaciers or the snows of the high neves are 

 permanently in contact with rock heated by the 

 sun. 



For their study it is only needful to collect these 

 films, these mucilaginous scums, and place them in 

 phials with the name of the locality where they w T ere 

 collected. Humid rocks, stones from brooks, or 

 aquatic plants should be brushed lightly with a small 

 camel's-hair pencil j or else delicately pass the brush 

 over the slime of the pond, over the organic felt, 

 dipping every time what the brush brings away into 

 a phial half full of water. At home leave the liquid 

 to settle, which has been decanted in order to observe 

 the sediment (see further, the manner in which pre- 

 parations are made for the microscope). At Geneva 

 it is not uncommon to find Nitzschia fusidium and 

 Navicula pellicitlosa in vases of water left in the rooms. 

 The water in which flowers have been standing often 

 contains Tabellaria jlocculosa and the different Gom- 

 phonemas, &c. At the bottom of the water-tanks of 

 our houses we nearly always find Cyclotella Kiit- 

 zingniana and operculata with the different Cymbellse, 

 together with many other lake species. In the plain, 

 it is during the months of March, April, and May 

 (in a word, at the end of winter and during spring) 

 that excursions to collect them alive are most fruitful. 

 During the height of summer and in autumn their 

 development partially ceases. In the elevated and 

 colder parts of the Alps we still find them abundantly 

 in the middle of summer, especially in the Alpine 

 lakes, or in the high torrents of the Jura. It is when 

 the torrents of the high Alps in winter flow limpid 

 from the glaciers that they are the most rich in 

 diatoms, even in water covered with ice. In summer,. ' 

 when the melting of the snows becomes rapid, and 

 when their water is muddied by the slush which it 

 carries, this richness of vegetation considerably di- 

 minishes. These are the results of observations 

 which I have been able to make during our winter 

 ascents with the Alpine Club. 



Sometimes, when I wished to obtain some par- 



