220 



KNOWLEDGE. 



[October, 1901. 



between plant and plant, but between plant and animal 

 — between the plant, indeed, and its wliole environment. 

 We thus find, in any selected area, a group of plants, 

 often differing widely in size and appearance, in mode 

 of growth and mode of dispersal, which nevertheless 

 foi-ms a strictly natural gi-oup, living together by reason 

 of their being" adapted to the particular conditions of 

 life which prevail. To such natural groups of the 

 vegetation the name of ;)7fl?i< nnforiatioiis has been given. 

 Such an association is often dominated by one or more 

 social species, which, gi-owing in great numbers, control 

 more or less the entire association. In other cases, 

 though some plants may be more abundant than others, 

 the association may be a commonwealth rather than a 

 monarchy. Let us glance at a few instances. On the 

 sandy sea-shore, vegetation is limited, but between the 

 reach of the waves on the one side, and the bent-covered 

 dunes on the other, a certain number of plants maintain 

 an existence. The Sea-Holly is here, with its leathei7 

 spiny foliage; the bushy succulent leafless prickly Salt- 

 wort; straggling plants of Orache ; the beautiful Horned 

 Poppy, with its grey downy foliage and splendid yellow 

 blossoms; the Sea-Rocket, also of bushy growth, with 

 tough stems and succulent shiny leaves ; and the Sea- 

 Spurge, which matches the Sea Holly and Honied Poppy 

 in the gi-ey tones of its foliage. What conditions have 

 detei-mined the selection of this group of plants? In 

 winter the sea washes right over these sands, and even 

 in summer the air is full of salt. Very few plants could 

 endure the salinity which pervades the place ; but these 

 particular species not only endure it, but cannot exist 

 without it; we find them nowhere but on the edge of 

 the sea. Next, the exposure here is very great. The 

 wind whistles along these bare sands, carrying spray, or 

 driving sand. The plants are all low, tough, bushy, 

 strong-rooted, suitable for withstanding gales. Heat and^ 

 drought are much felt here, and would prove fatal to 

 many species ; but all these have roots which strike deep 

 into the sand ; some, like the Salt-wort and Sea Rocket, 

 store up a large reserve sujiply of water and food in 

 their fleshy leaves; others like the Sea Holly, in their 

 long juicy undergi'ound stems ; and all provide against 

 too great transpiration, by means of a thick impenneable 

 skin, or a felt-like covering of hairs ; often accompanied 

 by a reduction in the number of stomata — the pore-like 

 cpenings of the leaves. Competition can play but little 

 part in the economy of this plant association ; there is 

 room for plenty more ; the abundant space allows each 

 plant to spread out, low and bushy. Human: inter- 

 ference is also at a minimum here, and animal depre- 

 dations likewise — though we note that the Sea Holly 

 and Salt-wort both provide for this contingency in the 

 production of a fomiidable array of thorns. The 

 dominant conditions ai-e undoubtedly the saltness 

 and the exposui-e ; these are the features of the environ- 

 ment that have chiefly determined the limits and 

 character of the plant association. 



Let us take an example of quite another kind — a wood 

 of Beech. Here the tall trees form the leading feature, 

 and dominate the whole association. The deep shade 

 cast by the foliage precludes a large number of plants 

 from trespassing here or enjoying tlie thick layer of 

 leaf-mould which has formed ; the few that have crept 

 in are spindly and sickly from their vain efforts to 

 obtain a sufficiency of light. But oven the denser por- 

 tions of the wood are inhabited by a variety of herbs, 

 and on examination, we find we may group these under 

 two heads : (1) spring flowers, such as Primroses, Wild 

 Hyacinths, Wood Saiiiclo. which by growth early in 



the season produce their leaves and blossoms before the 

 foliage overhead has yet screened out the sun and rain ; 

 and (2) saprophytic plants, such as the Bird's-nest 

 Orchis, which, instead of attempting to manufacture 

 plant-food by means of green leaves and sunlight, derive 

 their plant-food ready-made from the decaying vegeta.ble 

 matter, and thus ai'o enabled to flourish in the. gloomiest 

 recesses of the forest. In this case the Beech fonns a 

 strongly dominant species ; the subsidiary vegetation 

 being influenced chiefly by the limitation in the supply 

 of light. It is to be noted that the conditions which 

 exclude so many plants are advantageous or necessai^, 

 for the members of the association. Were the trees cut 

 down, the saprophytes would disappear at once, and most 

 of the other plants would seek a more sheltered habitat. 



Consider next the conditions prevailing in a pasture. 

 Here intense competition is going on ; every scrap of 

 the surface is densely covered with vegetation ; and in 

 addition, grazing animals are perpetually nibbling the 

 herbage, destroying the leaves and flowers. Note the 

 upright gi-owth of all the plants composing the asso- 

 ciation, and the naiTow leaves which most of them have 

 assumed, to get up into the light and air as far as 

 possible with the least expenditure of material. Several 

 species of grasses are here dominant species ; some 

 Composi.ffe, such as Daisies, Hawk-bits, and Cats-ears, 

 also appear well able to hold their own. There is also 

 an interesting group of semi-parasitic Scrojyhulariactce — 

 the Eyebright, Yellow-rattle, and Red-rattle, which, 

 when they get a chance, help themselves and hinder 

 their neighbours by fastening on the roots of the latter 

 and drawing therefrom plant food ready-made. Most>- 

 of the species are perennials, almost the only annvials 

 being the group last mentioned, which are certainly 

 advantageously circumstanced. The plants comprising 

 such an association must be very hardy species, able to 

 iiold their own in a keen struggle for room, and to 

 produce fresh leaves and perfect their flowers and fniit 

 in spite of the depredations of the sheep and cattle. 



Or t/O take, finally, the vegetation in the centre of a 

 great turf-bog. Here, again, we revert to conditions 

 entirely uninfluenced by man or by grazing animals. 

 We have again a strongly dominant species — the Ling. 

 Competing with it for dominion we may have one of 

 several others — the Bog Asphodel, or one of the sedges. 

 The supremacy vai'ies according to the degi-ee of mois- 

 ture. On the drier portions, the Ling easily holds its 

 own ; but where the peat is wetter, the Bell-Heather is 

 a dangerous rival ; and where water usually lies, the 

 Beak-rush holds sway. Compared with the flora of the 

 pasture, the variety of plant-life is very limited — per- 

 haps not more than] a dozen species in as many square 

 yards; very few plants can tolerate the peculiar con- 

 ditions which prevail — the intensely peaty soil, the soak- 

 ing spongy ground, and the competition of the over- 

 mastering Ling. 



THE WHITE NILE FROM KHARTOUM TO 

 KAWA. 



AN ORNITHOLOGIST'S EXPERIENCES IN THE SOUDAN. 



By Harry F. Wituerby, f.z.s., m.b.o.u. - 



IV.-- CAMPING AND COLLECTING. 



Whilst travelling up the river we had noted different 

 places where the country seemed most suitable for bifds, 

 and for making collections. Arriving at such a spot on 

 oiu' rctuiMi journey, if satisfied with our fonner clioice, we 

 selected the shadiest tree available under which to pitch 



