DESMIDIACE/K : DIATOMACE.^E 587 



supply. If the bottles be freely exposed to solar light, these little 

 plants will flourish, apparently as well as in their native pools ; and 

 their various phases of multiplication and reproduction may be 

 observed during successive months or even years. If the pools be 

 too deep for the use of the hand and the scoop, a collecting- bottle 

 attached to a stick may be employed in its stead. The ring-net 

 may also be advantageously employed, especially if it be so con- 

 structed as to allow of the ready substitution of one piece of muslin 

 for another. For. by using several pieces of previously wetted 

 muslin in succession, a large number of these minute organisms 

 may be separated from the water; the pieces of muslin may be 

 brought home folded up in wide-mouthed bottles, either separately 

 or several in one. according as the organisms are obtained from one 

 or from several waters ; and they are then to be opened out in jars 

 of filtered river water and exposed to the light, when the desmids 

 will detach themselves. 



The Diatomacese <>r Bacillariaceae. like the Desmidiacea?, are 

 simple cells, having a firm external coating, within which is included 

 an endochrome whose superficial layer constitutes a -parietal 

 utricle.' but their external coat is consolidated by sllf.f, the pre- 

 sence of which is one of the most distinctive characters of the 

 group, and gives rise to the peculiar surface-markings of its members. 

 It has been thought by some that the solidifying mineral forms a 

 distinct layer exuded from the exterior of the cellulose wall; but 

 there seems good reason for regarding that wall as itself inter- 

 penetrated by the silex. since a membrane bearing the characteristic 

 surface-markings is found to remain after its removal by hydro- 

 fluoric acid. The endochrome of diatoms consists, as in other 

 plants, of a viscid protoplasm, in which float the granules of 

 colouring matter. In the ordinary condition of the cell these 

 granules are diffused through it with tolerable uniformity, except 

 in the central spot, which is occupied by a nucleus ; round this 

 nucleus they commonly form a ring, from which radiating lines of 

 granules may be seen to diverge into the cell-cavity. Instead of 

 being briyht screen , however, the endochrome is a yellowish brown. 



. 



The principal colouring substance appears to be a modification of 

 ordinary chlorophyll ; it takes a green or greenish-blue tint with 

 sulphuric acid, and often assumes this hue in drying; but with it i^ 

 combined in greater or less proportion a yellow colouring matter 

 termed diatomin. which is very unstable in the light and fades in 

 drying. At certain times, oil-globules are observable in the 

 protoplasm; these seem to represent the starch-granules of the 

 Desmidiacece and the oil-globules of other protophytes. A distinct 

 movement of the granular particles of the endochrome, closely 

 resembling the cyclosis of the Desmidiacece, has been noticed by 

 Professor W. Smith in some of the larger species of Diatomacece, 

 such as Stirii-i'Un biseriata. Xitzschio wn/ftrts. and Campylodiscus 

 spiral-is, and by Professor Max Schultze in Coscinodiscits, Biddulphia, 

 and Rhizosolenia ; but this movement has not the regularity so 

 remarkable in the preceding group. 



The name of the class is derived from the ease with which the 



