GROWTH, ETC., IN THE DESMIDIACE^. 281 



to say that these ornaments are /o.s/ is mei-ely a facon de parley, as 

 a matter of fact they never grow. Cell-division becomes so con- 

 tinuous that there is not time for them to form on the young semi- 

 cells. 



Influence of Local Conditions. — In the determination of 

 the life-histories a great deal has been expected from the cultivation 

 of Desmids and Diatoms under artificial conditions in a laboratory 

 or aquarium. This does not appear to have answered even with the 

 Diatoms, which do not seem to be so acutely affected by their 

 surroundings, provided there is abundance of silica in the water 

 they inhabit. The Desmidiacea, on the other hand, are extremely 

 sensitive to very slight changes of temperature, wind, currents, 

 sunlight, the chemical condition of the water, the liability of their 

 habitat to dry up, and whether this takes place regulai'ly at a 

 certain period of the year, or irregularly, according to the raJnfalL 

 it is this sensitiveness and responsiveness to external stimuli which 

 are the cause of the immense number and variety of the forms- 

 included within any species. There is good reason to doubt 

 whether under the equable conditions of an aquarium the poly- 

 morphic forms would be reproduced. Each distinct form is the- 

 product of particular conditions, the tendency being to perpetually 

 reproduce that same form while the same conditions continue. 

 As confirmatory of this, it is interesting to note the experience of 

 G. S. West, mentioned in " British Freshwater Algse," on the 

 occurrence of certain polymorphic forms of Eremosphcera viridis, 

 reported by Chodat. " Specimens kept under cultivation for twO' 

 years," he says, " developed no forms other than globular daughter 

 cells." On the other hand I have observed here several distinct 

 forms of Eremosphcera viridis when growing under natural con- 

 ditions, and traced a connection with a good many more." 



Pause or Hesitation Points. — At any rate it is certain that 

 in different localities the same species may be regularly met with,, 

 here under this form and there under that. All the forms are 

 capable of going forward to perfection, and under other circum- 

 stances do so ; yet here and there they never seem to get beyond 

 a certain stage. All these polymorphic forms are transitory, mere 

 stages of gi'owth, however diverse in outline and markings they may 

 be, yet there is a stabihty about certain variations which gets 

 them the credit of being fully developed. This is due to a dis- 

 inclination on the part of the cell to go forward. For fresh develop- 

 ment it needs, as do all plants, an external stimulus from Nature, 

 and this is not always forthcoming, perhaps only rarely, except at 

 certain seasons of the year. The degenerate cell, therefore, simply 

 goes on reduplicating its own and still more degenerate forms over 

 a considerable period of the year. This disinclination to develop, 

 shown by the Desmid at any stage of its existence, has been takeri 

 to be an inability to proceed further, and what is merely a temporary 

 pause in their life-history has been elevated into the rule of Desmid 

 life. It is well known that the higher plants come to a standstill 



