424 PROTOPHYTA 



(auxospore or zygosperm) into two symmetrical halves with or without 

 the intervention of a period of rest. 



Still another mode of reproduction is described by Count Castracane 

 and by some other observers, in Mastogloia (Thw.) and a few other 

 genera, in the production of endogenous spores within the frustules. 



It will be seen that, notwithstanding the great abundance of diatoms, 



some important points in their life-history still remain unsettled. On 



the minuter details of the modes of reproduction, the spontaneous 



motion of diatoms and its causes, the structure of the sihceous cell-wall, 



and the chemical and physical properties of diatomin, the reader is 



referred to the very extensive literature of the subject ; only the most 



important memoirs are referred to below. The number of described 



species certainly exceeds 10,000 ; but this has been unduly increased 



by want of attention to the necessary variations in size in the same 



species. Not unfrequently diatoms form a gelatinous yellow scum 



on the surface of the water, or completely encrust submerged algae 



and other w^ater-plants ; they abound on the surface of \vet walls 



and rocks, and are not unfrequently present in the air. Some species 



are cosmopolitan ; the marine forms are especially remarkable for their 



size and beauty. Various deposits found on the surface of the globe, 



often of very considerable thickness, know^n as tripoli, ' Kieselguhr,' 



cSjc., consist almost entirely of the fossilised siliceous shells of diatoms, 



and they enter largely into the composition of a variety of earths 



used for manufacturing purposes. In some countries, such as China, 



Japan, Siberia, Lapland, cjcc, they form, cemented together by salts 



of lime, the edible earths which are mixed with meal to make a kind 



of flour. They occur also in large quantities in guano. 



As has already been stated, the position of the Diatomaceae in the 

 natural system is a point on which there has been much controversy. 

 Those who regard the mode of reproduction already described as a true 

 process of conjugation place them in the class of Conjugatae, near to the 

 Desmidiaceae, with which family they present many points of resemblance 

 in external form, phenomena of spontaneous movement, &c. ; and it is 

 possible that the diatoms may be derived from the desmids by retro- 

 gressive metamorphosis. But we are, on the whole, disposed to the 

 conclusion that they have a totally different origin ; their very wide dis- 

 tribution in time and space, the sharp differentiation of the family, and 

 the enormous number of species, favouring the view that they represent 

 a comparatively small ascent from an archaic type which has never 

 attained any higher degree of development. 



lUusirative genera: — Eunotia (Ehrb.), Diatoma (DC), Melosira 

 (Ag.), Gomphonema (Ag.), Navicula (Bory), Rhabdonema (Ktz.), 



