306 Yapp. — On Stratification in the Vegetation of a 
I have been to some extent guilty of arguing from the particular 
to the general : the differences, therefore, between the particular vegetation 
described in this paper and other types must not be lost sight of. For 
instance, the substratum, in the case of marsh vegetation, is unusually damp, 
and this may of course influence the humidity of the air in the lower strata. 
But it can scarcely be doubted that in all dense vegetation physical differ- 
ences more or less comparable to those described above exist at various 
levels. To take a single instance : Schimper 1 distinguished three layers of 
epiphytes in the primaeval forests of tropical America. The epiphytes 
of the upper layer, i. e. on the tree-crowns, are much more xerophytic 
in structure than those of the two lower layers in the forest. Schimper 
correlated this with the differences of light, and air moisture. It would, 
however, be of interest to determine the actual evaporation at various 
levels in a woodland, as well as in other types of vegetation. 
The Problems of Different Species. 
From the general results recorded in this paper it is obvious that the 
problems which confront any given plant must vary considerably according 
to the habit of growth of the species. It has been shown in an earlier paper 
that the subterranean parts of marsh plants place themselves at various 
levels, according to the degree of moisture appropriate to the species. 2 
We can thus speak of a distinct stratification of the roots, in addition 
to that exhibited by the shoots (cf. Text-figs. 4 and 6). The latter has 
already been discussed, but the former is also important. For instance, in 
the case of a very wet soil all except quite the upper layers are but poorly 
supplied with oxygen. It is to be noted that a very large number of 
marsh plants are ‘ surface rooted \ 
Now, apart from its specific structure and the length of its vegetative 
period, the physiological problems of any given plant will depend to a large 
extent on : — 
1. The depth of its root-system. 
2. The height above the soil to which its shoots reach. 
3. The layer of the vegetation in which its larger leaves are placed. 
It has been shown that different species vary greatly in all these 
respects. One has only to consult Text-fig. 4 to realize the very consider- 
able environmental differences of such plants as (say) Hydrocotyle on the 
one hand, and either Cladium or Phragmites on the other. 
Hydrocotyle has its leaves in the lower, more humid strata of the vege- 
tation ; its roots are practically on the surface of the damp soil ; and it 
is sheltered from the wind. Thus, not only are the diurnal fluctuations 
of both temperature and evaporation reduced to a minimum, but whatever 
1 Schimper (’88), chap, iii, pp. 89 et seq. 
2 Yapp (’08), pp. 68 et seq. 
