OF THE DIATOME.E. 103 



tered in previous secti-ons of this history of the Order, particularly in that 

 on their habitats (p. 75) ; yet, to make the directions complete, additional 

 details are necessary. 



AMiere Diatomese in the li\ing state exist in any considerable number, 

 they usually fomi a brilliant cinnamon, or sometimes an olive-brown film or 

 patch, and thereby become visible to the naked eye or to an ordinary lens, 

 adherent to various water- weeds, to decayed portions of wood, leaves, or 

 other floating substances, or as a patch on the mud at the bottom, or other- 

 wise floating on the surface of the pond as a scum or film. Besides such 

 positions and such collections, Diatomeae exist diffused more or less abun- 

 dantly thi^ough the water or in the mud itself (see p. 75 et seq.). 



When seen adherent to an aquatic plant, the process of collection is very 

 simple — by carefully gathering or removing the j^lant from the water and 

 washing it to detach the Diatomaceous fi^ustules, if these cannot be more 

 advantageously viewed whilst still adherent to its stem or leaves. So, too, 

 where, mostly in conjunction with other organisms, the Diatomeae float in 

 mass, like a scum on the siuface, nothing is easier than to lightly skim the 

 collection from the suiface. But when the layer of frustules reposes on the 

 surface, or is more or less intermixed with the mud, some additional pre- 

 cautions are required in theii' collection, unless indeed the film has sufficient 

 tenacity, by cohesion of its component frustules, as in the case of Schizonemeae, 

 to allow of its being raised en masse upon some thin flat instrument, a spoon 

 or spatula, insinuated beneath it. 



The general methods of collection applicable to the Desmidieae and other 

 minute Algae are equally so to the Diatomeae, whilst various modifications 

 will suggest themselves to the mind of every practical natui^ahst to meet the 

 vaiying circumstances under which he makes the collection. Mr. E,alfs has 

 kindly fui^nished us with notes on this point. He writes — " It is often 

 difficult to prociu'e clear specimens of those species which form strata on 

 mud ; most of them, however, can be obtained, tolerably free from the mud 

 on which they congregate, by the following method, which is applicable both 

 to those found in marine situations and to those gathered from the wayside. 

 When the water is somewhat dried ujd, if the finger be pressed upon the 

 stratum with a gentle force, the Diatomaceae wiU adhere to the finger, and 

 may then be removed by scraping them oif upon a piece of linen folded over 

 the edge of a tin box or of a knife ; by repeating this process, a sufficient 

 quantity can easily be collected. At fiLi^st, probably, a portion of mud, espe- 

 cially if very wet, will also be taken up ; but a little practice will soon show 

 the force requisite for places where the water is plentiful, and for those 

 where it is nearly dried up. Specimens thus collected can be prepared for 

 mounting with much less trouble than if gathered mixed with a large quan- 

 tity of dirt." 



^VTien it is mshed to capture frustules diffused in water, a piece of muslin 

 may be used as a filter, just as for Desmidieae, and the residue left upon 

 it examined as it is, or, if requii^ed, washed, to detach foreign matters mixed 

 wdth it. Where some admixture of mud is imavoidable, frequent washing of 

 the collected substance ^vill often suffice to separate sufficiently the silicious 

 frustules from the other particles — the heavier grains of sand sinking to the 

 bottom of the vessel, while the Diatoms are still suppended in the fluid ; and 

 on the other hand, the decayed organic and other matters, lighter than the 

 frustules, will remain in the supernatant liquid after the latter are precipi- 

 tated. Repeated careful decanting and washing may be aU, therefore, that is 

 required. 



Another method applicable to recent living specimens, dependent on the 



