402 



NATURE 



[August 25, 1898 



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. 



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 



Fig. 8.— Cross-section of leaf of Crowberry. The lower figures show on* 

 of the peculiar hairs and one of the stomates. Both are confined to the 

 inner, which is properly the under surface. 



to a minimum. A section of the leaf of Nardus, Aira flexuosa, 

 or Festuca ovina shows that the upper surface, which in grasses 

 bears the stomates, is infolded, 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 

 furrows with intervening ridges, and where the folding is par- 

 ticularly 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 power of rolling up may soon be lost by the leaves. Other 

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



Fig. 9. — Longitudinal section of root of Ling, {Calluna vulgaris), showing 

 mycorhizal filaments in outer cells. 



all stages the power of rolling up their leaves. Sesleria ceer- 

 ulea, 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, according to the state of the air. 

 The leaves of the true moorland grasses (Nardus, Aira flexuosa, 

 Festuca ovina) are permanently inroUed, and flatten out very 

 slowly and imperfectly, even when immersed in water for many 

 hours. 



Our moorland grasses are all arctic, and occur both in the old 

 and the new worlds ; Festuca oviiia is also a grass of the 

 steppes ; it is world-wide, being found in all continents, 



NO. 1504, VOL. 58] 



especially on mountains, and even reaching Australia and New 

 Zealand. 



It may seem paradoxical to count Ihe Rushes as plants which 

 are protected against drought, for they often grow in the wettest 

 part of the moor. They are common, however, in dry and 

 stony places, and their structure is completely xerophylic. The 

 leaves are often reduced to small sheaths, which wither early, 

 while the stems are green, and perform the work of assimilation ; 



Fig. 10. — Transverse section of leaf of Nardus stricta, showing permanent 

 in-rolling. 



or else, as happens in certain species, the leaves assume the 

 ordinary structure of the stem. The cylindrical form of the 

 Rush-stem is significant, for of all elongate solid figures the 

 cylinder exposes the smallest surface in proportion to its volume. 

 Moreover a cylindrical stem, without offstanding leaves, and 

 alike on all sides, is well suited, as Jungner points out, to the 

 circumpolar light, which shines at low angles from every quarter 



Fig. II. — Transverse section of leaf of Airajiexnosa. 



in succession. A Rush-stem is singularly dry, the centre being 

 occupied by an abundant pith of star-shaped cells, which 

 entangle much air. 



The Hair-moss (Polytrichum commune) of the moor has a 

 defence against sun and wind, which has been described by 

 Kerner. The leaf has wings, like an altar-piece, which can 

 open and shut. The assimilating surface occupies the centre, 



