Problem of Xeromorphy in Marsh Plants. 86 1 
while the sheltered leaves are more hygrophytic in character. Probably the 
chief danger to this species is the drying effect of wind, especially when 
coupled with a temporary desiccation of the surface layers of soil. 
Cladium Mariscus presents somewhat of a contrast. It is a wet-marsh 
plant, which is more pronouncedly xerophytic than Spiraea. Its roots 
grow in water-logged soil, and are indeed frequently entirely submerged. It 
is a relatively tall plant, and not only are the upper parts of its leaves 
exposed to wind and sun, but they are evergreen. Absorption, then, may 
possibly be impeded by such factors as lack of oxygen, toxic substances, 
&c., in the summer time, and by low soil temperatures in winter and spring. 
At the same time the leaves may be exposed to unduly severe transpiration 
conditions at any time of the year. Hence it is not surprising that, as 
is actually the case, Cladium should be more decidedly xerophytic than 
Spiraea. The comparison could readily be extended to include such diverse 
types as Hydrocotyle vulgaris, A ndromeda polifolia , &c., &c. 
To sum up, there is every reason to suppose that associations of bog 
and marsh plants are as much in harmony with their environment as are 
those occupying other distinctive habitats, such as the various woodland, 
desert, and other plant associations. But to arrive at this conclusion, it 
is necessary to take many factors into consideration. In all probability 
edaphic factors, climatic factors, the growth forms of the plants, with the 
resulting differences of exposure and shelter, the duration of the vegetative 
period, and possibly other factors as well, have all contributed to the produc- 
tion of a series of physiologically intricate, and at first sight paradoxical 
types of vegetation. 
Finally, it may be once more emphasized that the xeromorphic struc- 
tures in question are required to meet, not so much the everyday needs, as 
the extremes, possibly even only the occasional extremes, which the plants 
possessing them are called upon to face. 
Summary of Results. 
1. The present paper is an experiment in what may be called species- 
ecology. It is an attempt to see how far the intensive study of a single 
species, supplemented by observations on others, may help towards an 
explanation of the problem of xeromorphy in marsh plants. 
2. The species selected ( Spiraea Ulmaria ) is a £ dry-marsh ’ plant, the 
leaves of which are densely pubescent on the lower surface. This pubes- 
cence is subject to a kind of periodicity, and appears only under certain 
definite conditions. The chief rules governing the appearance of the hairs 
are as follows : (a) The seedlings, also all leaves formed during the first 
year, are glabrous, (b) On the erect flowering shoots of adult plants, there 
is a regular succession of glabrous, partially hairy, and completely hairy 
