March, 1913. 



KNOWLEDGE. 



99 



shaded woods, however, are more or less xerophilous in 

 structure, and grow high up on trees, whose crowns admit a 

 good deal of light. In general, xerophilous mosses are so 

 constructed that they rapidly absorb rain-water and dew, 

 and retain it for long, once it is absorbed. 



The majority of xerophilous mosses grow on rocks and 

 trees. They occur from the coast up to the high Alps, the 

 Grimmias and Andreaeas being the last representatives of the 

 cryptogamic vegetation seen in ascending the highest hills. 

 The effect of a dry continental climate is seen in general 

 scarcity of mosses rather than an increase in the proportion 

 of xerophilous species. The author then indicates the chief 

 adaptations shown by mosses for retaining moisture or 

 reducing transpiration. The chief xerophilous adaptations of 

 this kind in the sporogonium are absence or shortness of the 

 seta, causing the capsule to be sessile, or nearly so, and there- 

 fore enveloped by the upper leaves of the moss-plant; down- 

 ward curvature of the seta, causing the young capsule to be 

 plunged among the leaves — later the seta becomes erect for 

 spore-dispersal ; the presence of warty outgrowths or papillae 

 on the seta ; the presence of a large calyptra in some mosses 

 covering the entire capsule ; the hair-clad type of calyptra seen 

 in the Polytrichaceae and Orthotrichaceae ; the sheltered 

 position of the stomata, especially in species of Ortho- 

 trichum with furrowed capsules and the stomata in the 

 furrows, or in Polytrichum with stomata in the deep ring- 

 like groove between capsule and apophysis. 



The moss-plant itself, or gametophyte, naturally shows a 

 far wider range of xerophilous or drought-resisting characters. 

 These adaptations include, above all, perhaps, the cushion- 

 like habit, the plants being aggregated to form a spongy mass, 

 which holds water by capillarity ; hyaline hair-points on the 

 tips of the leaves; reduction in breadth of the leaf-blade, and 

 corresponding increase of the cylindrical midrib ; leaves more 

 or less hollowed out or rolled up, undulate, wrinkled, or 

 folded — all these arrangements providing cavities for the 

 retention of water by capillarity ; sheathing leaf-bases, as in 

 Polytrichum; papillae and mammillae (solid and hollow out- 

 growths respectively of the leaf-cells) ; presence of water- 

 storing tissue in the leaf ; thick-walled leaf-cells ; increase in 

 number of layers of the lamina to two or more ; strengthening 

 of the leaf-margin ; development of plate-like outgrowths or 

 lamellae on the upper side of the leaf ; presence of filamentous 

 outgrowths (paraphylls) and dense covering of rhizoids on the 

 stem ; hygroscopic movements of the leaves. 



ALPINE MOSSES AND LIVERWORTS.— Apart from 

 the familiar moss-carpet of woods, and the bogs largely made 

 up of bog-mosses (species of Sphagnum), and the woolly 

 fringe-moss (Rhacomitrium) on moorlands, various mosses 

 and liverworts play an important part in the vegetation of 

 certain areas, chiefly in alpine districts. 



Under the title " Anthclia: an alpine-arctic plant 

 association," Dr. W. G. Smith {Scottish Botanical Review, 

 April, 1912) gives an interesting account of a plant association 

 which is characteristic of the higher Alps, and which was 

 noted on Ben Lawers during the international phyto- 

 geographical tour of this country last summer. This associa- 

 tion is of special interest as one in which several liverworts 

 and mosses play the part of pioneers in colonising a substratum 

 which owes its origin in the first place to topography, and in 

 the second to the action of running water. Towards the 

 summit, where the grassy turf becomes more limited, and the 

 most conspicuous vegetation consists of the woolly fringe- 

 moss, sedges, and other mat-forming plants, there occur dark 

 patches, represented on the summit ridge by larger tracts of 

 almost black mossy crust. These patches and tracts are 

 known to Swiss botanists, two of whom were with the party 

 and pointed them out, as '' Schneetalchen," which may be 

 translated as " snow-flushes." 



The formation of these '"snow-flushes" has been traced 

 in Switzerland and Tyrol, where the rocks and soil, 

 uncovered by the melting snow, are often covered with 

 flowers, while the snow still lies a few yards away. As 

 melting proceeds the snow-water soaks through the 

 turf, forming a system of temporary watercourses in troughs 



of undulating ground, and along the foot of slopes and 

 escarpments, while the rain-water in summer tends to follow 

 the same course. On gentle slopes or flats and in depressions, 

 the force of the flow is not sufficient to carve out channels, as 

 happens on steep slopes, but the water wanders slowly through 

 the turf and deposits accumulated suspended matter as a 

 sediment. The snow-water carries the dust it gathers on 

 lying snow, mineral particles, and plant fragments, and 

 collects other materials in trickling over the surface ; snow- 

 dust may contain fifty per cent, of organic matter, and as this 

 with the mineral matter is laid down among remains of last 

 year's vegetation, a rich soil is built up. 



The vegetation of the snow-flush begins with flowerless 

 plants, and these may remain as the dominant vegetation. 

 Apparently the pioneer plant is the liverwort Anthelia, not 

 only on Ben Lawers and other Scottish mountains, but also 

 in the Alps, Scandinavia and Spitzbergen. This liverwort 

 lies close to the surface, and in the fresh, moist state forms a 

 bluish-green carpet, but in summer is often dry and dark 

 brown or almost black ; it is often coated with the threads of 

 a fungus, and these occur also in the rooting-hairs of the 

 liverwort — doubtless forming a mycorhiza or mutually bene- 

 ficial partnership between the two plants. Alpine species of 

 the moss-genus Polytrichum also occur in the snow-flushes; 

 these mosses can withstand periodical submergence, and soon 

 grow through the shallow deposits of sediment laid down, so 

 helping to bind these deposits into a humous turf, while their 

 close, compact growth also enables the tufts to withstand 

 periods of drought. The snow-flush is an open association 

 into which species of flowering plants from neighbouring 

 plant communities migrate — Dr. Smith gives lists showing that 

 several of these species are identical with those found in snow- 

 flushes in the Alps. 



On collating the accounts given by different observers, so as 

 to obtain an outline of the evolution of the snow-flush vegeta- 

 tion', we find that Anthelia, probably preceded by algae and 

 other lower organisms, forms a humous turf ; Polytrichum 

 follows later, and more or less takes the place of Anthelia ; 

 later still, Salix herbacea or Alchemilla assumes chief 

 place, and so on. The successive stages of vegetation 

 probably indicate stages in the evolution of the habitat, since 

 the later vegetation will tend to give it increased stability ; in 

 time the accumulation of sediment, humus, and vegetation 

 may be such that the snow-water is diverted to new situations, 

 where the sequence will begin over again ; and during the 

 various phases other species secure a footing, and flourish 

 well or ill according as the habitat suits them. The snow- 

 flush is, therefore, a series of migratory associations. 

 Crampton, whose important paper on the vegetation of 

 Caithness was recently summarised in these columns (" Know- 

 ledge," May, 1912, p. 187), has defined such formations as 

 follows (Scottish Botanical Review, 1912) : " Migratory 

 formations are of comparatively short persistence on the same 

 habitat, which sooner or later undergoes change or destruction, 

 with renewal elsewhere. Their associations tend to rapid 

 degeneration from plant invasion. All stages of progressive 

 successions are encountered." 



AIR-CHAMBERS IN LIVERWORT THALLUS.— The 

 Marchantiales, doubtless owing to the common occurrence of 

 the familiar Marchantia and Lunularia on the soil in flower- 

 pots, garden paths, and so on, and Fegatella beside streams, 

 are often taken as typical of the Liverworts (Hepaticae), though 

 in reality they form a relatively small division of that group, 

 the majority of Hepaticae being differentiated into slender 

 stem and thin leaves. At any rate, the thalloid Hepaticae 

 are usually selected by teachers of Botany as being the typical 

 forms, and Marchantia polymorpha is one of the most 

 frequently studied of plant types. As so often happens, even 

 the most familiar types repay renewed investigation by those 

 willing to take the trouble and to remain unbiassed by the 

 oft-repeated descriptions copied from text-book into text-book. 



The air-chambers which occupy the upper portion of the 

 tissue in the thallus of the Marchantiales, each chamber, in 

 typical cases, opening to the atmosphere by a pore in the 

 centre of its one-layered roof, were described by Leitgeb in 



