282 PROCEEDINGS OF SECTION D. 



in dry weather, but who has not observed the instant growth they 

 make under the stimulus of good rain. Aquatic life also has its 

 appropriate stimuli, and until these are supplied the Desmid will 

 simply remain in much the same condition, though slight changes 

 still go on. Each characteristic form, therefore, marks the place 

 for a pause in the development of the species, and in each locality 

 there is a tendency for this pause to remain constant at some 

 particular variation. 



I lately transported, from a small piece of sphagnum bog at 

 Coogee, a medley of Desmids and sowed them on a small isolated 

 piece of natural swampy ground at Auburn. Among them was 

 Eu. dideltoides, not found at Auburn. At Coogee, where there is 

 a generous current of cool water, this plant maintains itself full 

 grown, though I have once caught it young in a state like Eti. 

 ansatum, Ralfs, but in its new habitat, where water only lies after 

 rain for a few weeks at a time, it took on a new shape, the kink in 

 the side being accentuated to produce a figure like a form of Eti. 

 simiosum. Again at Collector, 250 miles up country from Sydney, 

 its prevailing young form was that described and figured as E. 

 iriangidum, Playf. {Jour. Linn. Soc, N.S.W., 1907, p. 171, PI. iii, 

 f. 7), which should stand in future as En. dideltoides var. triangulum. 

 The same Desmid, therefore, in different localities may have alter- 

 native life-histories, or at any rate different young forms become 

 prominent. 



Influence of Heat. — Rise in temperatme is the principal 

 cause of cell-division. This will be at once noted in gatherings 

 taken on a hot day from shallow water ; these are sure to contain 

 cells in all stages of division, and even the most mature of them 

 more or less degenerate. The same thing may be observed in 

 gatherings kept alive in glass jars during hot weather ; division 

 proceeds so rapidly and continuously that the characteristic size, 

 form and markings of the cells are entirely obliterated in the cells 

 produced. A gathering of Desmids was lately obtained in which, 

 among others, Micr. trnncata v. decemdentata was abundant. A 

 part of this gathering w^as placed in a small glass jar near a window, 

 where it caught the sun daily for a few hours. Examined after 

 some days, the original var. decemdentata could hardly be found 

 except in mixed forms, but quite a number of degenerate types had 

 come into existence, showing great debasement of form consequent 

 upon quickly repeated division. Not a trace of any of these forms 

 was originally present.^ 



The Desmids frequent shallow waters, and these in tropical 

 and sub-tropical countries easily become quite hot. Moreover they 

 abound in the tufty heads of water weeds, by which they are 

 generally elevated into the upper, and therefore warmer, strata of 

 their habitat. All this tends to promote multiplication of degene- 

 rate forms and to reduce all the species to their smallest and most 

 compact proportions. It is interesting to note that this very fact 



* iour. Lmn. Soc-, N-S-W., 1910, PI. xiv. 



