176 Transactions. 



way for future development, but is a distinct beginning and end in itself 

 — i.e., it is a plant formation closely related to but distinct from the later- 

 formed river-bed associations. All the same, it is always present, since 

 when destroyed at one spot it is being renewed elsewhere. 



This primary conmiunity may be designated the Epilobium association, 

 since one or other species of this genus is the first-comer, thanks to the 

 wind-borne seeds, their rapid germinating-power, and the quick develop- 

 ment of the seedlings. Also, there is present more or less of the mat- 

 forming Raoulia tenuicaulis. The characteristic species of Epilobium for 

 montane river-bed are the erect-branching E. melanocaulon and one or 

 more of the varieties of the mat-forming E. pedunculare, a polymorphic 

 species. 



As the bed becomes stable^i.e., as low terraces are built by the river 

 with their surfaces beyond the reach of the highest floods — another asso- 

 ciation makes its appearance, and there is now a gradual procession of 

 events, according to climate and altitude, from an association where mat 

 and low cushion plants dominate to low tussock grassland, shrubland, or 

 even forest. 



Coming now to the vegetation of the river-bed in close proximity to the 

 biolugical station — ^namely, that of the River Cass — we are face to face 

 with some apparent discrepancies in the above general remarks. This 

 river, which rises in the Craigieburn Mountains, is only some eight miles 

 in length. Its upper portion, flowing through a forest-clad gorge, is of 

 a more or less torrential character, and does not come within the scope 

 of this paper. The lower part of the river flows through its ancient flood- 

 plain, which at the widest part is some two miles across. Here the true 

 river-bed is remarkably broad in proportion to the water which it carries, 

 so that it is far more stable than the habitat in general, and in consequence 

 the primary and secondary plant-communities are not sharply defined, 

 and there are far more species on that part of the bed, still liable to flood, 

 than is usually the case in. such a habitat. AH the same, the actual pro- 

 cession of events is as detailed above, though it is not so evident as on 

 many river-beds. 



The vegetation of the unstable river-bed, now about to be described, 

 is not one association, but a combination of the true primary Epilobium 

 association and the succeeding Raoulia association. The most important 

 members are Epilobium melanocaulon, Raoulia tenuicaulis, and R. australis 

 var. On portions of the substratum not swept bare by water for some 

 time there may be tussocks of Festuca novae-zealandiae, and the introduced 

 Holcus lanatus, Rumex Acetosella, and Cerastium triviale. 



On river-terrace recently formed, but which is of sufficient age to have 

 enabled plant-colonists to become established, in addition to the species 

 already mentioned, the following are present : MneJilenbeckia axillaris 

 forming circular wiry interwoven mats, the introduced Sagina procumbens, 

 several varieties of Acaena inermis and A. microphylla forming rounded 

 mats, the introduced Trifolium repens, rosettes of Geranium sessiliflornm 

 var. glabrum, Discaria toumatou (here prostrate), Raoulia Haastii forming 

 dense green cushions, and the inti'oduced Hypochoeris radicata. On such a 

 young terrace the vegetation is liable to rapid destruction when attacked 

 by a stream which has changed its course. 



On long-establisbed terrace occupation by plants for a considerable 

 period has added humus to the soil, so that a more favourable station 

 for vegetation has gradually developed, thanks to the plants themselves. 



