276 Yapp . — On Stratification in the Vegetation of a 
of hydrophytic species side by side with species possessing xerophytic 
characters . 1 
But most authors admit that the whole question is still in a state 
of uncertainty. 
Now although this problem has attracted a good deal of attention, the 
methods of attack seem to have been incomplete. For instance no one 
appears to have really carefully studied the habits of the plants concerned, 
together with their relations to each other, and to the physical conditions of 
their habitats. 
The author has been engaged for some years on the study of marsh 
vegetation, in the hope that the results of a more detailed investigation of 
the plants, both in the field and in the laboratory, might bring us nearer to 
a solution of this, and possibly of other problems. The present paper 
treats of the structure of the vegetation of a marsh, and more particularly 
of the relations of the aerial shoots, both to each other and to some of 
the habitat factors which directly affect transpiration. It is intended 
in later papers to deal with other aspects of the subject of ‘ swamp 
xerophytes’. 
Stratification 2 of the Aerial Shoots. 
The field work which forms the basis of this paper was chiefly carried 
on at Wicken Fen, in Cambridgeshire, and a preliminary account of the 
Fen Vegetation has already been published . 3 The observations on the 
habits of marsh plants made there have, however, been supplemented 
by others on marsh vegetation in various parts of the country. 
In the first place it may be mentioned that, although the herbaceous 
vegetation is very mixed, and composed of many species, yet so many 
of the plants grow to about the same height that the facies of the vegetation 
is fairly uniform. At Wicken Fen the actual height of what we may term 
the £ general vegetation level ’, or ‘ general shoot-level ’, varies, according to 
the dominant species (and, of course, other factors) from some eighteen 
inches or two feet to four or five feet above the ground. 
Between this upper level of the vegetation and the soil are the aerial 
shoots of the plants. It will be instructive to consider the vertical distribu- 
tion of the transpiring organs of the different species. In doing so, we may 
1 Cf. also Warming (’96), p. 177. 
2 1 Stratification ’ is used here to signify the differences found in vegetation at different vertical 
levels. The various stages may be called strata or layers. The German equivalents are Etagen 
(Schimper (’88), p. 99), Schichten or Stockwerke (Warming (’96), p. 117)- Clements ('05), p. 280, 
uses the phrase ‘ vertical zonation ’, in the sense that stratification is used here. He defines zonation 
(p. 274) as ‘the practically universal response of plants to the quantitative distribution of physical 
factors in nature ’. But as Clements also includes under the term zonation, the altitudinal and 
latitudinal differences in vegetation, it seems to me that in practice it will be better to use another term 
to describe this common phenomenon of ‘ vertical zonation ’. 
3 Yapp (’08), pp. 61-81. 
