858 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
the greatest water capacity. It would therefore seem probable that an 
appreciable amount of the water actually present will not be available for 
absorption by the plant. With a view of determining what the actual per- 
centage of non-available water may be in moorland soils, Crump 1 has 
conducted a number of interesting wilting experiments. He calculates that 
certain plants (e. g. Eriophorum angustifolium ) are unable to absorb more 
than about 48 per cent, of the total water present. 
The above are some of the more important suggestions that have been 
made with respect to edaphic factors. But, as both Livingston and Dach- 
nowski point out, our knowledge of the chemistry of bog water and bog 
soils is still very incomplete. 
ii. Atmospheric factors . Certain authors deny the importance of these. 
E. g. Dachnowski, 2 referring to the xerophytic characters of bog plants, 
says, ‘ None of these features are correlated with atmospheric influences.’ 
Elsewhere, however, he seems to admit that they may play a part. 
On the other hand, both Kihlman and Goebel (see 2 above) emphasize 
the importance of drying winds, especially when accompanied by coldness 
of soil 
The present author 3 has shown that even in the comparatively dwarf 
vegetation of a marsh, great and constant differences of relative humidity 
exist in the different aerial strata. Thus the leaves of plants which occupy 
the lower strata are, on e. g. dry and windy days, exposed to transpiration 
conditions far less severe than leaves growing in the upper strata of the 
vegetation. More recently Dachnowski 4 has obtained similar results in 
the bog vegetation of Cranberry Island. The position, then, of the tran- 
spiring organs of the various plants must be of considerable importance. 5 
Indeed, a slow absorption of water can only be prejudicial in so far as it is 
liable to be accompanied by excessive transpiration from the aerial parts. 
Another factor that has to be taken into account in this connexion 
is the length of the vegetative period of the different species. So far, for 
instance, as the danger arising from limitation of absorption by reason 
of low soil temperatures is concerned, it is obvious that, in temperate 
latitudes, this will be experienced to the greatest extent by evergreen plants 
during the winter or spring time. Schimper 6 was the first to emphasize 
the connexion between the coldness of the soil and the xerophytic structure 
of the leaves of evergreen trees and shrubs in temperate regions. Fruh and 
Schroter 7 similarly attribute the need for such structure in the plants 
1 Crump (’12). 
2 Dachnowski (’ll, second paper), p. 34. Cf. also (’10), p. 338. 
3 Yapp (’09). 4 Dachnowski (’ll), p. 146. 
6 Kearney (’01), p. 442, in his survey of the Dismal Swamp Region, points out that the tall, 
reed-like plants of freshwater marshes protect the smaller, more hygrophytic forms against excessive 
transpiration. 
6 Schimper (’90), pp. 15-17. 
7 Fruh and Schroter (’04), p. 15. 
