On Stratification in the Vegetation of a Marsh , 1 
and its Relations to Evaporation and Temperature . 2 
BY 
R. H. YAPP, M.A., F.L.S., 
Professor of Botany in the University College of Wales, Aberystwyth. 
With Plate XX, and eight Figures in the Text. 
I T has long been known that many marsh and bog plants possess 
so-called xerophytic characters. But although various theories have 
been put forward from time to time, to account for the existence of these 
‘ swamp xerophytes ’, none of them can be said to be either established, or 
even generally accepted. Without dealing with the historical aspect of the 
question in detail, one or two of the more important theories may be 
mentioned. 
Kihlman 3 regards these xerophytic structures as necessary in high 
northern latitudes, where, he points out, root-absorption may be hindered 
by the coldness of the wet or even frozen soil, at the same time that strong 
drying winds tend to increase transpiration. Goebel 4 adopts a similar 
explanation in the case of the swamps of the Venezuelan Paramos. 
Schimper 5 regards swamp soils as being in many cases ‘ physiologically 
dry ’, owing to the high percentage of salts, humic acids, &c., in solution. 
Stenstrom 6 takes a similar view. 
Other authors think that these xerophytic structures are primitive 
characters, which were originally developed under habitat conditions very 
different from those obtaining at present, and now survive only in the more 
stable species. Schwendener 7 has come to this conclusion as a result 
of his study of the distribution of the Carices : while Clements 8 and others 
hold the same view. These authors lay stress on the occurrence in nature 
1 This is equivalent to the German Flachmoor. 
2 Some of the results given here formed the subject of a paper read before Section K of the 
British Association, Dublin Meeting, 1908. 
3 Kihlman (’90), pp. 107 et seq. 4 Goebel (’91), Teil II, p. n. 
5 Schimper (’90), pp. 635 et seq., also (’08), p. 15. 6 Stenstrom (’95), p. 71. 
7 Schwendener (’89), p. 73. 8 Clements (’05), p. 126. 
[Annals of Botany, Vol. XXIII. No. XC. April, 1909.] 
U 
