632 Professor L. C. Miall, [Feb. 18, 



maintains that tlie mycelial mantle is the chief means of absorption 

 from the peaty soil, and that the tree or shrub has come to depend 

 upon it. The known facts render this interpretation probable, but 

 thorough investigation is still required. In some cases at least the 

 j)lant can be gradually inured to the absence of a mycelial mantle. 

 I have repeatedly planted crowberry in a soil devoid of peat. It 

 generally succumbs, but when it survives the first year, it maintains 

 itself and slowly spreads. Microscopic examination shows that the 

 roots of crowberry grown without peat contain no mycelial filaments 

 or very few. The special function of the fungus may be to reduce 

 the peat to a form capable of absorption as food by green plants. It 

 is likely that the fungus gains protection or some other distinct 

 advantage from the partnership. Most of the species of green plants 

 which have the mycelial mantle are social. It is obvious that the 

 fungus will be more easily propagated from plant to plant, where 

 many trees or shrubs of the same species grow together. 



Fig. 9. — Lougitudinal section of root of Ling (Calluna vulgaris), sLowing 

 mycorhizal filaments in outer cells. 



The grasses of the moor are marked xerophytes with wiry leaves, 

 whose look and feel tell us that they have adapted themselves to 

 drought and cold by reducing the exposed surface to a minimum. A 

 section of the leaf of Nardus, Aira flexuosa or Fesfcuca ovina shows 

 that the upper surface, which in grasses bears the stomates, is 

 in-folded, and sometimes greatly reduced. Advantage has been taken 

 by these grasses of a structure which was apparently in the first 

 instance a provision for close folding in the bud. The upper stomate- 

 bearing surface is marked by furiows with intervening ridges, and 

 where the folding is particularly complete, both furrows and ridges 

 are triangular in section, and the leaf, when folded up longitudinally., 

 becomes an almost solid cylinder. In the grasses of low, damp 

 meadows, the j)ower of rolling up may soon be lost by the leaves. 

 Other grasses, which are more liable to suffer from drought, retain in 

 all stages the povrer of rolling up their leaves. Sesleria cserulea, for 

 instance, which covers large tracts of the limestone hills of Yorkshire, 

 can change in a few minutes from closed to open, or from open to. 

 closed, accordinpf to the state of the air. The leaves of the true 



