CHAP, xviii EFFECT OF NON-LIVING COVERING 75 



parts to certain other plants, particularly to arctic, alpine, and desert 

 plants. 



A fact that has long been known, and already mentioned on p. 24 of 

 this work, is that the old, dead leaves remain attached in great numbers 

 to the branches of sub-glacial plants, and thus envelop them with dense 

 coverings, whose closeness is further increased b}' the production of con- 

 densed short branches. This is evidently a result of the circumstance 

 that the processes of disintegration and decay take place extremely 

 slowly in the cold climate where bacteria and fungi do not thrive ; it is 

 of utility to the plant in obstructing transpiration. Nature ensheaths 

 plants just as a gardener in preparation for winter mulches sensitive 

 forms. 



Certain species growing upon dry rock or similar arid spots are, in 

 like manner, enveloped by remains of old twigs and leaves ; in this case 

 it is lack of moisture, not of heat, that arrests the disintegrating action 

 of fungi and bacteria. That the plants concerned derive any benefit 

 therefrom cannot be generally asserted, though it is probable. It is 

 possible to conceive that these old plant-parts serve partly as a protection 

 against transpiration, and partly as organs for the absorption and reten- 

 tion of water. In this connexion attention may be directed to the tunic 

 grasses,^ also to the envelopes formed by the leaves and roots on the 

 stems of the Velloziaceae, and by the roots in Dicksonia, and some other 

 ferns.- 



CHAPTER XIX. EFFECT OF A LIVING VEGETABLE 



COVERING ON SOIL 



Every kind of covering formed by vegetation acts upon the physical 

 relations in soil ; and the denser, taller, and longer-lived the vegetation 

 is, the more powerful is its action. Forest therefore acts most powerfully ; 

 and for this reason the vegetation clothing the ground in forest, on the 

 one hand, and the plants forming the high-forest, on the other hand, are 

 subject to entirely different physical relations. The effects partially 

 agree with those wrought by inanimate coverings : 

 I. The temperature of the soil is modified. 



A vegetable covering screens the soil, and therefore decreases the 

 action of the sun's heat. But a vegetable covering is a very effective 

 radiator of heat. Fluctuations of temperature, both diurnal and annual, 

 are less considerable ; compared with soil clad with vegetation, bare 

 soil is warmer by day and colder by night, warmer in summer and colder 

 in winter. The maxima of temperature are much higher, but the minima 

 only httle lower in bare soil than in shaded soil (clad with vegetation), 

 so that the mean temperature of the latter soil is lower by, at any rate, 

 one or two centigrade degrees in forest than in bare soil. The temperature 

 at the surface of forest soil in Central Europe, according to Ebermayer, 

 is rarely higher than 25 C. Inside forest the dead covering of course 

 contributes to increase these effects. 



' Hackcl, 1890; Warming, 1892 a. ' Warming, 1893; sec also Section III. 



