GROWTH, ETC.. IN' THE DESMIDIACE^. 283 



proves their salvation in drought. The drying up of the water, by 

 the consequent gradual rise in temperature, automatically brings 

 them into the best condition for resisting desiccation. If the heat 

 of the water had tended to promote development to their highest 

 proportions the drought would find them at their largest, with the 

 maximum surface to be attacked by the heat and with the broadest 

 possible pores to be protected from evaporation. 



It seems to be still considered doubtful whether the Desmids 

 can survive the desiccation of their habitat. From considerable 

 experience of swampy patches of ground which dry up periodically 

 I can state unreservedly that despite their dehcate appearance 

 there is no class of organism better qualified to withstand drought 

 under natural conditions. I have on many occasions taken fine 

 gatherings of Desmids from ground which had only a few days 

 before been revivified by rain, having previously been dry for 

 months under the fierce heat of an Australian sun. The mucus 

 with which the cells surround themselves is quite able to keep the 

 drought at bay for a considerable period. Besides, to organisms 

 of their microscopic proportions the sohd ground is like perforated 

 zinc, and although apparently quite dry, yet within the interstices 

 into which they can descend there is still sufficient moisture in 

 many cases to keep minute plants alive. 



Occasional Variations. — Besides the line of variations which 

 constitutes the normal life-history of any species, each of its com- 

 ponent types is the centre of a cluster of forms which are not in the 

 direct line of development, but are the product of special con- 

 ditions. These may be called " occasional " variations. They 

 have no place 'iit the normal life history, but they are not for that 

 reason to be considered as developmental culs-de-sac. They grow 

 out of one form and into another and develop onwards towards the 

 final form, producing a secondary life-history more or less irregular. 

 These occasional variations are, of course, merely temporary ; but 

 still having occurred once there is always the likelihood of their 

 occurring again. It is unfortunate when, as sometimes happens, 

 one becomes the type of a species as the form first found. This 

 class of variation is useful from the way in which the cells some- 

 times combine the shape of one form with the markings of another, 

 thus establishing their connection with the same species. 



Senile Forms. — Akin to these occasional variations are what 

 Turner, in Alg. of E. India, has termed senile forms, a very suitable 

 name, which may well be perpetuated. If a cell gets into a back- 

 water of life, into circumstances, that is, which are inimical to 

 robust growth, a feeble life often continues to go on in the cell, but 

 the only outcome of it is to cause incrassation and yellowing of 

 the membrane generally, the puncta or scrobicuhe being thus 

 brought greatly into prominence. The rounded outline of the .semi- 

 cell at the same time becomes angular, and even thespines, if present, 

 become incrassate. Such are Pen. cucurhitinum var. pachydermuni 

 (Playf.), Jour. IMin. Soc. N.S.W., 1907, PI. ii, f. 6; Ar. gihberulu^ 



