816 Yapp . — Spiraea Ulmaria , A., <2;^/ z'A Bearing on the 
association. But in the meantime it is probable that light may be thrown 
on many ecological problems by the intensive study of the ecology of 
selected species. 
The present paper, which is intended as a further contribution to our 
knowledge of c swamp xerophytes \ 1 is an attempt to see how far the study 
of a single species may help towards an explanation of xeromorphy in 
marsh plants. The species chosen for this purpose is the common Meadow- 
sweet, Spiraea Ulmaria , Linn., the leaves of which, as is well known, are 
covered beneath by a dense white felt of hairs. In this paper special atten- 
tion is devoted to this characteristic pubescence, which is usually regarded 
as a device for depressing transpiration. 
§ 2. Hairiness in Marsh Plants. 
Though aquatic phanerogams, as a class, are remarkable in being for 
the most part devoid of hairs, the latter are by no means uncommon on the 
leaves or stems of marsh plants. Indeed, I have noted the presence of 
a greater or less degree of hairiness in upwards of thirty of the commoner 
British species of marsh plants. No doubt even this list could be con- 
siderably augmented. In some cases the aerial parts are only slightly 
hairy, e. g. in Angelica sylvestris, Hydrocotyle vulgaris, Valeriana sambuci- 
folia, Phragmites communis , Molinia coerulea , &c. In others the hairiness 
is more pronounced, as in Epilobium hirsutum , Cnicus pr a ten sis , Lysimachia 
vulgaris, Mentha aquatica, & c. In a few cases the lower surface of the 
leaves is covered with a dense felt of hairs : such is the case in Spiraea 
Ulmaria, Salix spp., &c. 
It may be remarked here that hairiness occurs most frequently in 
those species of marsh plants which either grow in the drier parts of marshes, 2 
or else have their aerial parts more or less exposed to the effects of drying 
winds. 3 
§ 3. Distribution of Certain Hairy Species of Spiraea. 
The genus Spiraea 4 comprises some 127 species, which are spread 
over the temperate and colder regions of the northern hemisphere. Of 
these, the majority appear to be glabrous or nearly so, though others are 
hairy to a greater or less degree. A few species have dense pubescence on 
the under surface of their leaves ; these fall naturally into two groups. 
In the first, which belongs to the § Ulmaria , are Spiraea Ulmaria, L., 
vS. palmata , Thunb., and N. vestita, Wall. These are closely allied Old 
1 Cf. Yapp (’08, two papers), (’09), and (TO). 2 e. g. Spiraea Ulmaria. 
3 e. g. Phragmites communis , Viburnum Opulus , Salix spp., &c. 
4 In the sense used by Linnaeus, Bentham and Hooker, and others, cf. Wenzig (’88). Other 
authors, probably rightly, separate the several sections of Spiraea as distinct genera, e.g. Focke (’94). 
