Problem of Xeromorphy in Marsh Plants. 855 
tive, says in this connexion : ‘ Finally, it may be noted that there are not 
only moors inclining to xerophily, but also others leaning rather to hydro- 
phily, and that, in addition to the structural types and forms of leaves men- 
tioned, there are others which apparently show no signs of xerophily, and 
cannot be shown to be in harmony with this habitati 1 
Several a priori arguments may be urged against the view advocated 
by Schwendener and Clements. The widespread occurrence of the pheno- 
menon ; the existence of varying degrees of xeromorphy in different types 
of marsh and bog vegetation, these being correlated with marked differences 
of soil, &c. ; the fact that many of these species ‘ can grow both on 
extremely dry, warm soil, and on extremely cold, wet soil ’ ; 2 and finally, 
the probability that xeromorphy, unless advantageous, would actually 
handicap marsh plants in their competition with other species — all suggest 
that such xerophytic devices are in some way related to a need imposed on 
the plant by the nature of the habitat. Further, the argument from proxi- 
mity of position, which has been so frequently employed, depends on the 
tacit assumption that plants growing together are necessarily under the 
same conditions. This is most certainly not the case. Indeed, the present 
author believes himself to have shown that, so far as the water supply 
of marsh plants is concerned, few of the species have to face precisely 
the same set of physiological problems . 3 
Various authors have discussed the question of whether a bog is to 
be regarded as a hygrophytic or xerophytic habitat. In this connexion 
it may be pointed out that only in the more extreme cases, and particularly 
(so far as xerophytes are concerned) those in which the plant associations 
are open, can habitats 4 as such be said to be markedly hygrophytic or 
xerophytic. In the great majority of closed, mixed associations, the 
mutual relations of the various species are such as to profoundly modify the 
general character of the habitat for each other ; some plants being exposed, 
others sheltered, and so on . 5 In this way, according to the growth forms 
of the species, and in varying degrees according to the general characters of 
the habitat and of the vegetation, the same habitat may be xerophytic to 
one species and hygrophytic to another. This is probably to some extent 
what Warming had in mind when he wrote the passage quoted in the intro- 
duction to this paper. The emphasis here laid upon this point of view 
is not out of place, for there is some danger that it may occasionally be lost 
sight of in the comparative study of vegetation in its broader aspects. 
This is especially so in view of the stress laid on the generally accepted 
ecological classification of plants into xerophytes, mesophytes, hydrophytes, 
1 Warming, 1. c., p. 196. The italics are mine. 2 Warming, 1. c., p. 194. 
3 Yapp (*09), pp. 306-8. 
4 Using the term in its broad sense, i. e. to indicate the general physical environment of plant 
associations, rather than merely of individual plants. 5 Cf. Yapp, 1. c., p. 279. 
