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waterlogged soil. Miss Stopes also, in describing “ The Colonisation 
of a Dried River Bed,” 1 mentions that Spiraea Ultnaria, though 
growing in great numbers on the bank, did not begin to invade the 
river bed until the second year after the diverting of the stream. 
By this time the bed was almost dried up. 
Possibly two factors at least are concerned in this limitation 
so commonly found in nature :— 
(1) . Competition. The moisture conditions under which a 
species is usually found, may be those in which it can most 
successfully resist the attacks of competing species. Some plants 
however, when once established, are able to resist attack, even if 
the physical conditions change, much more than others. Thus 
Phragmites, with its deeply placed rhizomes, and spear-like spring 
shoots, which can force their way up through the overlying plants 
with comparative ease, will hold its own for a long time, even if the 
superficial layers of the soil become dry. Plants of a different 
habit of growth however, such as e.g. Alisina Plantago, succumb 
much more readily. 2 
(2) . It is probably decided in many cases during the early 
stages; i.e. germination of the seed and growth of the seedling. 
Obviously, if the conditions are such as are very unfavourable to 
the young plant, there is but a poor chance of its surviving to the 
adult state. 
It would be interesting to put this matter to the test of 
experiment. 
It was mentioned earlier that, although invasion of dry places 
by “ wet plants ” was the more obvious, yet a reciprocal invasion by 
the “ dry plants ” did occur. We have seen that many of the Fen 
plants have creeping underground parts; but a tufted or caespitose 
habit is not uncommon. Molinia coerulea is one of the most striking 
examples of this. A good many Carices have a similar habit, while 
not infrequently, mixed tufts of y uncus obtusifiorus and Carex sp. 
are to be found in the wetter parts (Text-fig. 15). Mosses appear on 
these tufts, and form a dense growth binding the dead leaves, 
&c., together. Particles of soil and other debris may be washed in, 
and gradually the level of the clump is raised. When this is 
sufficiently high and dry, it forms a convenient germinating place 
for seeds of other plants, which are thus able to establish themselves 
above the damper parts of the soil. Seedlings of Ly thrum, Spiraea, 
1 Stopes, New Phytologist, Vol. II., 1903, pp. 188 and 191, 
a Stopes, l.c., pp. 188 to 192. 
