210 



KNOWLEDGE 



[Skptemiieb, 1902. 



Iiools art' filled with Sphagnum, and licr« and there with 

 the Bojj-boaii, Mt'iiyavthen trifollnfu ; the edges of the 

 ].<)ols are luxuriously eushioned with a variegated growth 

 of Sphaijiium. and these edges are the favourite haunt of 

 two of those wonderful fly-devouring Sundews, Dronera 

 roliindi/olia and D. (imjlica. In the shallower pools, or on 

 bare mud, the White Beak-rush, Rhynchospora alba, grows 

 in tufts, and its farther-creeping relative, R. fiisca, fills 

 portions of the pools with an erect growth of bright green 

 stems and brown inflorescence. These, with some subsidiary 

 mosses and lichens, complete the flora of our bog. 



The conditions under which this plant-group lives are 

 remarkable. The sj) mgy crust is perennially saturated 

 with water, which circulates very slowly. In consequence 

 the soil is badly aerated, and the plant-remains which 

 form the crust do not become thoroughly oxidized, and 

 soluble humus compounds remain in solution in the 

 water. The plants can with difheulty absorb by their 

 roots water charged with these substances, and thus it 

 comes to pass that while the bog is physically very wet, 

 physiologically it is very dry. In the midst of plentv the 

 plants are actually starving. It is, indeed, a true case of 

 " Water, water everywhere. 

 Nor any drop to drink." 

 E.xactly the same difficulty, it may be remarked, occurs in 

 salt marshes, where the water is charged with sodium 

 chloride. In both cases the plants meet it in the same 

 way, by checking transpiration, and thus saving up the 

 water which they absorb. Thus they assume characters 

 similar to those displayed by plants of deserts and dry 

 I)laces — xerophytic characters, to return to a term which 

 we have had occasion to use before. These are seen in the 

 sinallness of the leaves of the Heaths, for example, as 

 well as in the curious backward-rolled character of their 

 leaves, as well as of those of the Cranberry, Crowberry, etc. 

 In other cases the leaves are protected by a thick im- 

 pervious cuticle or skin, or by a close growth of hairs. 

 Another peculiarity in the conditions under which the 

 bog flora grows is that in the water-logged soil there are 

 no bacteria present, which are so useful in bi-eaking up 

 the complicated nitrogenous compounds contained in the 

 dead plant matter ; hence nitrogen as a plant food is 

 scarce. This may help to explain why the Sundews, which 

 are essentially bog-plauts, have hit upon the extraordinary 

 manner of obtaining nitrogenous food for which they are 

 famous — the capturing, killing and digesting of small 

 animals. 



The flora of our bog has one peculiar point of interest — it is 

 one of the few plant-associations now to be found which is 

 absolutely uninfluenced by ubiquitous man and his works. 

 Over almost the whole of our islands the surface is under 

 his control ; even on the mountain-to2)s his sheep nibble 

 the herbage and influence the flora, but here not even the 

 sheep venture ; the flora, the relative abundance of its 

 constituent species, their dimensions and form, are purely 

 natural. The fauna is natural likewise. A few Hares 

 here find a seciu-e retreat. The lonely Curlew makes its 

 nest among the Ling, and the alert little Merlin selects for 

 its breeding-haunt some spot where a mound or high tuft 

 of moss furnishes it with a look-out tower. But the 

 leading inhabitant of the great bogs, and the only one 

 which conspicuously affects its flora, is the Black-headed 

 Gull. This bird, selecting a remote piece of bog with 

 numerous pools, forms breeding colonies containing some- 

 times thousands of nests. The effect on the bog flora is 

 disastrous. Owing partly to the tramping, and probably 

 more to the guano, ]ilant after plant succumbs, until from 

 the most thickly ]io]>ulated spots all are gone. In their 

 place there springs up a vegetation less exclusive in its 

 tastes and more tolerant of disturbance and of manuring. 



The seeds of these plants presumably come mostly 

 adhering to the birds' feet, from the pastures and cultivated 

 lan.l where the Black-headed Gull mainly forages. Tali 

 clumj)8 of rushes spring up and grasses of several kinds, 

 and with them Buttercups and Daisies, Chickweed and 

 Groundsel, and many other weeds — a complete change 

 of flora such as we seldom find except in connection with 

 that arch-meddler, man. 



We wander back to the edge of the bog, where the turf- 

 cutters have, in the course of half a century, effected quite 

 an appreciable nibbling into the vast brown vegetable 

 mass. The section expo.sed by their operations is 

 interesting. For a few feet from the surface the peat is 

 loose and spongy and dries to a yellow colour, and in it we 

 can detect plentiful remains of the plants which still 

 flourish on its surface —stems of Heathei-, the far-creeping 

 rhizomes of the Cotton-grass, and much Sphmjnum. 

 Below that the peat becomes more decayed, darker in 

 colour and denser in texture, till at length it is a compact 

 brown vegetable mud. In these lower layers are numerous 

 great stumps of trees — Fir and Oak — with far-spreading 

 roots. The lowest layer of black peat rests with startling 

 contrast on a band of pure white marl, which in its turn 

 is succeeded by a tough grey stony clay, whicli we recognise 

 as the weathered surface-layer of the limestone drift, or 

 Boulder clay, that is so thickly spread over the Central 

 Plain of Ireland. Here, then, is a series of deposits which 

 bridge over the great gap of time between the Ice Age 

 and the present day — perhaps thirty thousand, perhaps 

 sixty thousand years. Let us see what light the plant- 

 remains which may be found in these deposits can throw 

 on the successive floras that have occupied the surface 

 during this vast period ; thus we may link the present 

 with the past, and learn something of the history of our 

 existing vegetation, and of its ancestry. 



During the Glacial period, ice covered a great portion 

 of our country. The clays and gravels, full of ice- scratched 

 blocks, often yield also marine shells, and these are largely 

 of northern and arctic type, betokening very cold seas. 

 Have any plants of this rigorous period survived, and 

 what do they tell us ? We may take in answer one of the 

 interesting series of deposits discovered by the late Mr. 

 Bennie, in the neighbourhood of Edinburgh. In a cutting 

 at Corstorphine, lying on Boulder clay, the following beds 

 occurred in ascending order*: — 



1. Coarse grey sand, with stones. ^ The residue of the 



2. Fine laminated clay. ) Boulder clay. 



3. Layers of fine clay, mud, or silt, with leaves and seeds 



in thin layers, or scattered. 



4. Lake marl — a calcareous mud, crowded with the 



ordinary lake shells, and felted with stems of water 

 plants. 



5. Ochrey sand and gravel, six to seven feet thick, with 



little or nothing organic. 

 It is this bed 3 to which our attention must especially 

 be directed. The plant-remains, worked out by Mr. 

 Clement Keid, make the old flora which clothed Corstor- 

 phine Hill spread itself again before us. We see low 

 Willows and Birches and Brambles clustering here and 

 there, and while some of these still flourish in the district, 

 others are now restricted to the mountain-tops, and others 

 a^ain have retreated to within the Arctic Circle. The 

 Mountain Sorrel and Mountain Dryas, now alpine in their 

 distribution, are here too, and many plants of moors and 

 bogs, such as Sedges, Club-rushes, Bog-bean. Marsh 

 Andromeda, and Crowberry. These no doubt grew around 

 the marshy edges of the shallow pools, in which the plaut- 



* Bennio : " Arctic Plants of the Old Lake Deposits of Scotland." 

 Ann. Scott. Nat. Hist., January, 1894. 



