Problem of Xeromorphy in Marsh Plants . 859 
of a Hochmoor partly to difficulties of absorption, and partly to the 
prevalence of the evergreen habit. 
A Tentative Hypothesis. 
Much remains to be done before the causes which have contributed to 
the production of xeromorphy in bog and marsh plants can be stated with 
exactitude. In particular, our knowledge of the physical chemistry of 
peat soils is still very incomplete. It is also uncertain as yet to what extent 
the stomata of marsh plants are capable of normal movement . 1 Again, the 
osmotic properties of the cell-sap of marsh and bog plants would be worth 
investigating. But the problem is gradually becoming less obscure. As 
seen above, many possible factors have been suggested by different authors, 
each factor in turn being assigned a position of paramount importance. 
Probably the truth is that a number of factors, both edaphic and 
atmospheric, are operative. Further, it is probable that while some factors 
are of greater importance than others, the importance of any given factor 
will vary according to circumstances. As regards edaphic factors, the 
general trend of evidence, a good deal of it experimental, renders it increas- 
ingly probable that the soils of peat-marshes and bogs are to a'certain extent, 
if not physically, at least physiologically dry to plants. But the factor, or 
combination of factors, which renders absorption difficult is not the same 
in all cases. For example, low soil-temperatures probably play a part 
in far northern latitudes, at high altitudes, and in the case of evergreen 
plants during the winter-time. But the same factor may be entirely 
inoperative in the case of plants which only produce leaves during the 
summer-time ; and especially in the bogs and marshes of warmer lati- 
tudes. Again, the presence of toxic substances or the lack of oxygen 
(whether this acts directly, or merely by oxidizing injurious compounds) 
may possibly affect chiefly the more deeply rooted plants . 2 On the other 
hand, plants rooted on or near the surface may secure abundant oxygen, but 
are exposed to the danger of physical drought if the water table is 
lowered to any considerable extent. Further, it is probable that chemical, 
and perhaps also physical, differences between the soils of bogs and marshes 
(i. e. Hochmoore und Flachmoore) respectively, are largely responsible for 
the fact that xeromorphy is more prevalent in the plants of the former than 
in those of the latter (but see below). Doubtless in many cases two or 
more soil factors, often in different combinations, co-operate . 3 At the same 
time, we know little of the ultimate causes which limit the absorption 
of water by the roots. 
But edaphic factors alone cannot suffice to account for the fact that 
xerophytic and hygrophytic species so frequently occur side by side in 
1 Cf. Darwin (’98), p. -5-5.2, and the literature there cited. Also Friih and Schroter (’04), p. 14. 
2 Cf. Yapp (’08), pp. 74-5. s Cf. Warming (’09), p. 195. 
