MICROSCOPIC FOKMS OF VEGETABLE LIFE. 273 



they are, properly speaking, but repetitions of the same variety by a pro- 

 cess of simple multiplication, really representing in their entire aggregate 

 the one plant or tree that grows from a single seed. 



272. DIATOMACE^E. These, like the Desmidiace'ae, are simple cells, 

 having a firm external coating, within which is included an 'endo- 

 chrome' whose superficial layer constitutes a ( primordial utricle:' but 

 their external coat is consolidated by silex, the presence of which is one 

 of the most distinctive characters of the group, and gives rise to the 

 peculiar surface-markings of its members ( 277). 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 regard- 

 ing that wall as itself interpenetrated by the silex, since a membrane 

 bearing the characteristic surface-markings is found to remain after its 

 removal by hydrofluoric acid. The 'endochrome' of Diatoms consists, 

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

 coloring 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 bright green, however, the 

 endochrome is of a yellowish-brown. The principal coloring 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 is combined in greater or less proportion a yellow 

 coloring matter termed phycoxanthin, which is very unstable in the light, 

 and fades in drying. 1 At certain times, oil-globules are observable in the 

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

 diaceae (261) and the oil-globules of other Protophytes (229). A 

 distinct movement of the granular particles of the endochrome, closely 

 resembling the cyclosis of the Desmidiaceae ( 262), has been noticed by 

 Prof. W. Smith 2 in some of the larger species of Diatomaceae, such as 

 Surirella Useriata, Nitzschia scalaris, and Campylodiscus spiralis ; and 

 by Prof. Max Schultze 3 in Coscinodiscus, Denticella, and Rhizosolenia ; 

 but this movement has not the regularity so remarkable in the preceding 

 group. 



273. The Diatomacece seem to have received their name from the 

 readiness with which those forms that grow in coherent masses (which 

 were those with which Naturalists first became acquainted) may be cut or 

 broken through ; hence they have been also designated by the vernacular 

 term ' brittle- worts.' Of this we have an example in the common 

 Diatoma (Fig. 173), whose component cells (which in this tribe are 



1 A full account by M. Petit of recent Chemical and Spectroscopic investiga- 

 tions on the endochrome of Diatoms, will be found in " Journ. of Roy. Microsc. 

 Soc.," Vol. in. (1880), p. 680. 



2 The account of the Diatomacece, here given, is chiefly based on the valuable 

 "Synopsis of the British Diatomaceae," by the late Prof W. Smith; of which, 

 and of its beautiful illustrations by Mr. Tuffen West, the Author has been enabled, 

 tosmake free use by the liberality of Messrs. Beck. In the sketch he has given of 

 the Systematic arrangement of the group, however, he has followed the Classifi- 

 cation of Mr. Ralfs (Pritchard's " Infusoria," 4th edition). A more recent Classi- 

 fication proposed by M. Paul Petit will be found in the " Monthly Journal of the 

 Microscopical Society," Vol. xviii. (1877), pp. 10. 65. The new Monograph of the 

 group by Prof. Hamilton Smith (U.S.), announced as forthcoming, will doubtless 

 supersede all previous descriptions of it. 



3 " Quart. Journ. of Microsc. Science," Vol. vii. (1859), p. 13. 



18 



