PALMELLACEJE. 307 



Such split hairs (as Dr. Eedfern first noticed) may always be 

 found in a shaving brush which has been for some time in use. 

 Those should be selected, which have thin split portions so 

 closely in contact, that they appear single until touched at their 

 ends. When the split extremity of such a hair touches the 

 glass slide, its parts separate from each other to an amount pro- 

 portionate to the pressure; and, on being brought up to the 

 object, first pushed to the edge of the fluid on the slide, may 

 generally be made to seize it. Supposing that we wish to select 

 certain particular forms, from a Diatomaceous sediment which 

 has been obtained by the preceding processes ; either of the two 

 following modes may be put in practice. A small portion of 

 the sediment being taken up in the dipping- tube, and allowed 

 to escape upon the slide, so as to form a long narrow line upon 

 it, this is to be examined with the lowest power with which 

 the object we are in search of can be distinguished (the erector 

 and draw-tube, 43, 44, will here be very useful); and when 

 one of the specimens has been found, it may be taken up, 

 if possible, on the point of the hair, and transferred to a new 

 slide, to which it may be made to adhere by first breathing on 

 its surface. But if it be found impracticable thus to remove the 

 specimens, on account of their minuteness, they may be pushed 

 to one side of the slide on which they are lying ; all the remain- 

 der of the sediment which it is not desired to preserve, may be 

 washed off; and the objects may then be pushed back into the 

 middle of the slide, and mounted in any way that may be desired. 

 194. Palmellacece. To the little group of Plants which is 

 ranked under this designation, those two genera belong, which 

 have been already cited as illustrations of the humblest types of 

 vegetation ( 150, 152); and the other forms which are asso- 

 ciated with these, are scarcely less simple in their essential 

 characters, though sometimes attaining considerable dimensions. 

 They all grow either on damp surfaces, or in fresh or salt water ; 

 and they may either form (1) a mere powdery layer, of which the 

 component particles have little or no adhesion to each other, or 

 they may present themselves (2) in the condition of an indefinite 

 slimy film, or (3) in that of a tolerably firm arid definitely 

 bounded membranous "frond." The first of these states we 

 have seen to be characteristic of Palmoglcea and Protococcus : the 

 new cells which are originated by the process of duplicative sub- 

 division, usually separating from each other after a short time ; 

 and even where they remain in cohesion, nothing like a frond 

 or membranous expansion being formed. The " red snow/* 

 which sometimes colors extensive tracts in Arctic or Alpine re- 

 gions, penetrating even to the depth of several feet, and vege- 

 tating actively at a temperature which reduces most plants to a 

 state of torpor, is generally considered to be a species of Proto- 

 coccus ; but as its cells are connected by a tolerably firm gela- 

 tinous investment, it would rather seem to be a Palmella. The 



