Problem of Xeromorpliy in Marsh Plants. 865 
occupying other distinctive habitats. In other words, xeromorphy is of 
physiological utility to those bog and marsh plants which exhibit it. It 
may be regarded as directly related to the needs imposed on the plant 
by the special nature of the habitat. But in each case the special regulatory 
devices are required to meet, not so much the everyday needs, as the 
extremes, possibly even only the occasional extremes which the species has 
to face in nature. 
The environmental factors of a bog or marsh habitat are complicated. 
No single factor can be regarded as of sole importance in determining 
the need for xeromorphy. The hypothesis is put forward that, as suggested 
by so many authors, the soil is to some extent physiologically dry. The 
degree of physiological drought varies in different cases (e. g. bog soil is 
doubtless drier to plants than marsh soil), as do the factors or combination 
of factors which occasion it. But edaphic factors alone cannot account for 
the fact that hygrophytic and xerophytic species often live side by side 
in nature. Here atmospheric factors become important. Granted that 
the characters of the substratum are such that absorption of water is 
slow, then the atmosphere will play a differentiating role. A retarda- 
tion of absorption is only prejudicial when accompanied by an excess of 
transpiration. On the whole the more hygrophytic species, or the more 
hygrophytic stages or portions of other species (e. g. seedlings, or the 
lowest leaves of herbaceous plants), occupy the lower humid strata of 
the vegetation. Op the other hand, the more xerophytic species, or the 
more xerophytic parts of others, are those which grow under more 
severe transpiration conditions ; e. g. evergreen plants, and those occupy- 
ing the drier, more exposed strata of the vegetation. 
Thus, in all probability, edaphic and climatic factors, the growth forms 
of the various species, with the resulting differences of exposure and shelter, 
the duration of the vegetative period, and possibly other factors, all play 
their part in determining the need or otherwise of special devices for 
regulating transpiration in swamp plants. In short, after making due 
allowances for the general nature of the habitat, the whole matter really 
resolves itself into a consideration of the different problems of individual 
species. 
The work embodied in this paper was carried out chiefly at Aberyst- 
wyth, but, in addition, work was done at certain times at the Cambridge 
Botanical Laboratory and the Jodrell Laboratory, Kew. In addition, there- 
fore, to various acknowledgements made in the course of this paper, I wish 
to express my thanks to Professor A. C. Seward, and to Lieut.-Col. D. 
Prain, for permission to use these laboratories, respectively. Also to 
Mr. L. A. Boodle, for kindly providing facilities at the Jodrell Laboratory, 
and for several helpful suggestions and criticisms ; to Mr. R. I. Lynch, 
3 L 
