Aug.,"! Kelly, Plant Distribution in the Healesville District. 55 



1914 J » 



that the point of issue was hke the Hp of a basin, at which the 

 lake, when in flood, overflowed — probably into another lake 

 covering the Yarra flats below. The wearing away of the soft 

 rock was inevitable, and left a depression like the waist of an 

 hour-glass, through which the Yarra now flows. As this 

 opening was wearing the bed of the lake was filling up, until 

 the water was so shallow that, seeking levels, it cut channels 

 in the mud, forming river-beds which changed from time to 

 time as various obstacles blocked the course, or the level varied. 

 The lake is now often partly reproduced at time of flood. The 

 old river-beds, still plainly observable on the flats in the form 

 of depressions and billabongs, are first filled up ; then the 

 waters, spreading, often reach the old shores and there lap and 

 recede as they did of yore on a much greater scale. As the 

 waters receded vegetation appeared. 



The course of vegetative progress may be roughly recast by 

 examining the growths in the present streams and the various 

 billabongs and branches in ascending order from those with 

 permanent water to practically dry beds, proceeding from 

 plankton to benthos. Series after series of unicellular and 

 unattached multicellular plants, fresh-water algae, and mosses 

 which grow on stones in the running water in the upper 

 feeders, such as Fissidens rigidulus and the wet-ground moss, 

 Rynchostegium muriculatum, were probably among the first 

 forms. In the side washes of the lake, that at low water were 

 still, grew Potamogeton, Lemna (Duckweed), and in the shallower 

 moving water the long trails of Triglochin procera, with Ottelia 

 and Valhsneria, which adjust themselves to varying depths. 

 As the water found an outlet and gradually subsided, associa- 

 tions of Myriophyllum variifolium and various Ranunculi 

 appeared, with Typheta of Typha angustifolia and the pan- 

 phyton Arundo phragmites — forms, some of them wide apart 

 in evolution, but the ready representatives of an existent flora. 

 The habit of the yellow-flowered gentian, Limnanthemum 

 cxaltaium, is peculiar, in that it does not form a part of, but 

 has risen superior to, the Limnea formation, and occupies — 

 sometimes almost entirely by itself — isolated small indentations, 

 which may be permanent pools or merely moist saucers. As 

 the waters still further receded, raised portions of the old bed 

 became comparatively dry land, and were strewn by seeds 

 from all sides, many of which found suitable soil and environ- 

 ment. Thus were formed, in addition to the Typheta, Junceta 

 and Scirpeta in the remaining swamps, whilst the drier parts 

 selected such portions of the invading flora as were adaptable. 

 All these portions have since received into their formations 

 and associations a large number of acclimatized and naturalized 

 aliens. Some small swamps along the railway line are almost 

 wholly tenanted by exotic mint, Mentha pulegium. Pennyroyal. 



