Desmidiacese 



379 



in that plane containing the two longest axes. The other important positions 

 are the vertical (or end) view and the lateral (or side) view. 



Desmids are subject to some amount of variation, but only within certain 

 limits, and one of the most extraordinary 

 facts concerning these unicellular plants 

 is the constancy of the ornamenta- 

 tion of the cell-wall. Variations 

 occur mostly where the conditions have 

 allowed of very rapid multiplication 

 by cell-division, such prolific increase 

 resulting in peculiar, and certainly 

 abnormal, physiological conditions. 

 Similar conditions may supervene in 

 pure cultures with consequent modifi- 

 cation of specific characters. 



In pure cultures of Diatoms, which are 

 much more easily obtained than in the case 

 of Desmids, similar loss of specific characters 

 is far more noticeable, but it has been shown 

 experimentally that if these degenerate in- 

 dividuals are transferred to fresh culture- 

 solutions in which the physiological conditions 

 are more normal, the proper specific characters 

 quickly reappear during subsequent divisions. 



Fig. 239. A, a variety of Triploceras vertl- 

 cillatum Bail., from Australia, x 416. 

 B, portion of a filament of a Burmese form 

 of Micrasterias foliacea Bail, in which the 

 toothed polar lobes of the cells are not so 

 firmly interlocked as usual, x 433. 



Our knowledge of the geographical 

 distribution of Desmids, although very 

 incomplete, has advanced consider- 

 ably in recent years. We know now 

 that the family as a whole exhibits geographical peculiarities of a more 

 striking character than those shown by any other group of Green Alga3. 

 Although numerous species are ubiquitous there are many others confined to 

 definite land-areas of the earth's surface. Thus, one is able to discriminate 

 between definite Indo-Malayan types, African types, American types, Arctic 

 types, and so on. 



The only continental area from which Desmids are practically absent is 

 the Antarctic continent 1 , in which the conditions of existence are more severe 

 than in the Arctic areas and would soon prove fatal. The chief reason for 

 the absence of Desmids, however, is probably the great distance of the Ant- 

 arctic continent from the nearest land. This may be fully realized when it is 



1 Only one Desmid is known from the Antarctic continent, viz. : a small species of Cosmarium 

 closely allied to C. Ciicurbita Breb. This was recorded and described by Fritsch ('12) under the 

 name of ' Penium sp.' 



