856 Yapp . — Spiraea Ulmaria, L., and its Bearing on the 
& c. Such a classification is admittedly useful, but it should not be for- 
gotten that it is all a matter of degree, and that communities composed 
entirely of marked xerophytes, &c., are probably comparatively rare. 
Thus, though climatic and edaphic factors are the most important, in that 
they determine the general character of the vegetation, the inter-relation- 
ships of the various species also play a definite, if less conspicuous part, 
even in the case of relatively dwarf vegetation. But all this does not 
explain the paradox that certain of the plants rooted in the wet soil of 
marshes and bogs apparently need to exercise economy in respect to their 
water supply. We must therefore consider the theories put forward by 
authors who regard the xeromorphy of these plants as induced by the 
nature of the habitat. 
It is at once obvious that either atmospheric or edaphic factors, or 
both, may be concerned. The present writer’s researches have dealt mainly 
with the former, while most other observers have confined their attention to 
the latter. 
i. Edaphic factors may be taken first, the following comprising the 
more important suggestions put forward : 
1. Actual physical drought. Volkens 1 thought the important factor 
was the drying of the superficial layers of peat during summer. He pointed 
out that in some moorland and marsh habitats the soil is wet all the year 
round, while in others the upper layers become dry in summer. Studying 
the Carices in particular, he concluded that the more xerophytic species are 
largely confined to the second class of habitats. Davis 2 and Burns 3 have 
come to a similar conclusion from their observations on the lowering of the 
water table during droughts in North American peat-bogs. 
2. The lozv temperature of wet soils may, it is suggested, check root 
activity, and so reduce the absorption of water. Kihlman 4 regarded this 
factor as of great importance in northern regions, especially when coupled 
with drying winds. Goebel 5 came to the same conclusion with regard 
to the mountainous parts of Venezuela. Transeau 6 found, as the result of 
laboratory experiments, that a cold substratum caused a reduction in size 
of both roots and leaves, and induced the formation of xerophytic characters 
in the latter. The effect was more marked when, in addition, the soil was 
badly aerated. He is of opinion that substratum temperatures are of prime 
importance in northern latitudes, but that they are inadequate to account 
for xerophytic structures in bog plants in Southern Michigan. 7 
3. Paucity of oxygen in the soil. Transeau’s experiments showing 
the effect of a deficiency of oxygen have already been quoted. Again, 
1 Volkens (’ 84 ), p. 24. 
3 Burns (’ll), p. 119. 
2 Davis (’ 06 ), p. 160. 
4 Kihlman (’ 90 ), p. 107 et seq. 
6 Transeau (’ 06 ), p. 22 et seq. 
5 Goebel (’ 91 ), Teil ii, p. 11. 
7 Transeau, 1 . c., p. 36. 
