86o Yapp . — Spiraea Ulmaria , Z,., and its Bearing on the 
nature. It is here that atmospheric factors become important. Granted 
that the characters of the substratum are such that the rate of absorption 
of water is slow, then the atmosphere will play a differentiating role. A re- 
duction in the amount of water absorbed does not injuriously affect the 
plant, provided that transpiration is reduced in the same ratio. The latter 
is effected in one of two ways, according to the position of the aerial parts 
of the plants, (i) For the taller plants, the shoots of which occupy the 
upper strata of the vegetation, and are therefore liable to exposure to 
strong winds, insolation, &c., xeromorphy may well be a necessity; and 
even more so if the plants are evergreen. (2) On the other hand, the 
leaves of the smaller plants occupy the lower strata, where they are 
protected from wind, and transpire into a constantly humid atmosphere. 
As is to be expected, these lower strata do actually contain the more 
hygrophytic species, e. g. Hydrocotyle vulgaris , &c., and also the hygrophytic 
seedlings and lower leaves of other species. Conversely, the more xerophy tic 
species, e. g. Cladium Mariscus, or parts of shoots (cf. Spiraea Ulmaria), 
are those occupying the most exposed positions. 
So far as atmospheric factors are concerned, the less dense the vegeta- 
tion, and the larger the proportion of plants of an evergreen habit, the more 
xerophytic may the vegetation be expected to be. In this country these 
two conditions obtain to a greater extent in bog than in marsh vegetation. 
It is therefore not surprising, seeing that atmospheric and (probably) also 
edaphic factors are in general more favourable, that the vegetation of peat- 
marshes should contain a larger proportion of hygrophytic species than that 
of bogs ; and, conversely, that the latter should support a greater number 
of xerophytic types. 
The general nature of the habitat is of course important, but having 
made due allowance for that, the whole matter really resolves itself into 
a consideration of the different problems of individual species . 1 And if 
this be so in any given marsh or bog, much more is it true when we come 
to compare different types of swamp vegetation and different latitudes and 
climates. 
As an example, two species of marsh plants may be briefly compared : 
Spiraea Ulmaria is a plant of the drier parts of marshes. Its roots 
grow in moist but not water-logged soil. Probably they rarely experience 
lack of oxygen, and absorption in the well-aerated soil is unlikely to be 
much impeded by toxic substances. The vegetative period is comparatively 
short (about March to October), so that coldness of soil will scarcely play 
a very important part in these latitudes. On the other hand, it is liable to 
suffer from actual drought through lowering of the water table. The aerial 
parts have been discussed at length in this paper, and it has been shown 
that the exposed leaves exhibit a definite if slight degree of xeromorphy, 
1 Cf. Yapp (’09), pp. 306-8. 
