ON THE COIiSICAN FIR. 223 



strength of the soil over which it grows, but also of increasing 

 it in large measure; this property it possesses in common with 

 the spruce and silver firs, the Scotch and the Weymouth pine 

 (P. strohiis). Later on, about the twentieth to thirtieth year, 

 unless recourse has already been had to the axe, it naturally 

 thins itself out. The struggle between the stems for the mastery 

 is not of very long duration, though not as short as in the case 

 of the Scotch fir; it can bear more shade than the latter, and 

 when the stems are partly overshadowed by their stronger neigh- 

 bours, they are more tenacious of life and hold out longer. Such 

 a struggle for life may best be noticed in forests of spruce or of 

 silver lir, and, in consequence, in such woods there is a greater 

 number of stems per unit of area, while at the same time diffe- 

 rence in diameter between the strongest and weakest classes is 

 more striking than in the pine forests. While the natural pro- 

 cess of thinning out is going on, a covering of mosses (mainly 

 varieties of Hypnum) becomes formed under the favourable in- 

 fluence of the subdued light cast on the ground. This moss 

 supplies the place of the former most abundant fall of leaves; it 

 protects the soil from the heat of the sun and from the winds ; 

 it absorbs and retains moisture; when the lower parts of the 

 stalks decompose, they give to the soil all the constituents of the 

 leaves, though not in the same proportion.* But if the Timius 

 be fixed for too high an age, the number of trees per acre de- 

 creases, the amount of sunshine now falling on the soil is not 

 favourable to the development of the moss, it withers and disap- 

 pears, its place being usually taken by varieties of grasses and 

 other plants which exhaust the supply of humus without mate- 

 rially improving the soil. The vegetable mould yielded by some 

 plants is in fact detrimental to the growth of the next generation 

 of the forest trees. For example the humus arising from Ceno- 

 myce (Cladonia) rangiferina and several other Liehcncs is like 

 dust; Calluna, Erica and Vaccinium, on decomposing, form a 

 vegetable mould rendered unsuitable on account of its containing 

 tannin and wax. Such kind of humus ought to be removed 

 before sowing or xDlantiug operations have commenced. They 

 announce to the forester that the soil is, at least at the surface, 

 impoverished. It is only on good soils (where it may be observed 

 the pines do not thin themselves so rapidly, and the covering of 

 moss remains longer), that shade-loving, or at least shade-bearing, 

 trees or shrubs with light seed find their way into the forest, and 

 aid in retaining and improving the quality of the soil. 



Like other conifers, with the exception of the Yew {Taxus lac- 

 cata) and the larch {Larix Eitropact), the timber of the Corsican 

 fir has no vessels but simply canals, like pores, which contain a 



* Proveil in 1851 by Vonhausen of Giessen, who analysed Hijpnum splcndcns, 

 H. imrum, an^k II. tcnnariscinuiy . 



