600 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



that are produced by the process of binary division. Hence a very 

 considerable latitude is to be allowed to the limits of species, when the 

 different forms of Diatomacece are compared ; and here, as in many 

 other cases, a most important question arises as to what are those 

 limits a question which can only be answered by such a careful 

 study of the entire life-history of every single type as may advan- 

 tageously occupy the attention of many a microscopist who is at 

 present devoting himself to the resolution of the markings on 

 diatom- valves, and to the multiplication of reputed species by the 

 detection of minute differences. 1 



This formation of what are termed anxospores as serving to 



augment the size of the 



A B IJ cells which are to give 



origin to a new genera- 

 tion takes place on a 

 very different plan in 

 some of those filamentous 

 types, such as ^Mosim 

 (fig. 444, A, B), in which 

 a strange inequality 

 presents itself in the 

 diameters of the differ- 

 ent cells of the same 

 filament, the larger ones 

 being usually in various 

 stages of binary sub- 

 division, by which they 

 multiply themselves 

 longitudinally. Accord- 

 ing to the observations 

 of" Mr. Thwaites (loc. 

 FIG. 446. Conjugation of Epithemia turgida: A, cit.), these also are the 

 trout view of single frustule ; B, side view of the c ^ ^ f 



same; C, two frustules with their concave surfaces Products of a kind ot 

 in close apposition; D, front view of one of the conjugation between the 

 frustules, showing the separation of its valves E, adjacent cells of the Or- 

 F, side and front views after the formation of the j j i 



zygospores. dmary diameter, taking- 



place before the comple- 

 tion of their separation. He describes the eiiclochrome of particular 

 frustules, after separating as if for the formation of a pair of new cells, 

 as moving back from the extremities towards the centre, rapidly 

 increasing in quantity and aggregating into a zygospore (fig. 447, 

 No. 2, , I>, c) : around this a ne\v envelope is developed, which mayor 

 may not resemble that of the ordinary frustules, but which remains 

 in continuity with them ; and this zygospore soon undergoes binary 



See on this subject a valuable paper by Prof. W. Smith ' On the Determination 

 of Species in the Diatomacece,' in the Quart. Journ. of Microsc. Science, vol. iii. 

 855, p. 130; a memoir by Prof. W. (ire^orv 'On Shape of Outline as a Specific 

 ('h-u-aeter ot ]>if<,i/mceee,' in Traits, of Microsc. Soc. 2nd series, vol. iii. 1855, 

 p. H>; and the Author's Presidential Address, in the same volume, pp. 44-50; 'On 

 Na/oicula crassmervis, Frustulia saxonica, and N. rhomboides, as Test-objects,' by 

 W. H. Dallinger, M,,i,t/,/,/ M ><>. Jowrn. l7ii, vol. xvii. p. 1; also an Additional 

 note on the identity of these, by the same Author, ihiil. p. 173. 



