308 Yapp — On Stratification in the Vegetation of a 
scarcely to be wondered at that, for example, reed swamps should retain 
their peculiar physiognomy, even in warm climates. 1 
As mentioned at the beginning of this paper, some authors have 
regarded the fact that swamp plants both with and without xerophytic 
structures may grow side by side, as evidence that such characters cannot 
be a necessity to those species which possess them. Warming 2 also, in dis- 
cussing the whole question of swamp xerophytes, concludes by pointing 
out that there are some swamp plants which apparently have no xerophytic 
characters, and which cannot be brought into agreement with such a 
habitat. He instances Hydrocotyle , Caltha , &c. 
Whether such xerophytic structures are or are not present-day neces- 
sities to their possessors, I think it has been shown that any argument 
drawn from mere proximity of position, without reference to the varying 
physiological problems of the different species, is entirely inconclusive. 
Summary of Results. 
1. The primary aim of these researches is to throw light on the problem 
of ‘ swamp xerophytes \ The present paper, which deals with one aspect 
of the subject, describes the structure of the vegetation of a marsh. In 
particular, the relations of the aerial shoots, both to each other and to 
the external factors which directly influence transpiration, are discussed. 
2. In considering the vertical distribution of the transpiring organs 
of the various species, the plants in question are grouped into some five 
ecological types (p. 277). These types depend to a large extent on the 
relative positions, on the stem, of the larger leaves. It is shown that while 
rhythmic variations in the size of leaves is common to all the species, 
yet specific differences result in the attainment of the maximum leaf-size 
at different vertical levels, in various cases (cf. Text-figs. 1, 4, and 6). 
3. This difference of ecological habit results in a marked stratification 
of the vegetation, which varies in height from about two to five feet. But 
the strata or layers merge into one another more, and are therefore less 
distinct, than the very obvious vegetational strata of a forest. The sub- 
terranean parts also exhibit a stratification no less marked than that of the 
aerial shoots (Text-figs. 4 and 6). 
4. Evaporation as a means of determining the extent to which the 
atmosphere promotes transpiration under various conditions is discussed 
(p. 282). Also the relation of evaporation to other meteorological factors 
(p. 301). An evaporimeter is described (see Text-fig. 3), which is really 
1 cf. Warming (’96), p. 176. This question will be considered further in a later paper. It is 
merely mentioned here on account of a difficulty raised by Warming. He accepts the explanation 
of Kihlman and Goebel, with regard to the ‘ swamp xerophytes ’ of high northern and alpine regions, 
but remarks that it will not suffice in the case of warmer climates. 
2 Warming, 1. c. 
