790 THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



found in the deposits of the Mesozoic and Tertiary Periods. The number of extant 

 species identified hitherto amounts to about 1800. 



4.— THE DISTRIBUTION OF SPECIES. 



Distribution of Species by Oflfslioots.— Distribution of Species by Fruits and Seeds.— Limits of 

 Distribution. — Plant-Communities and Floras. 



THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



When the dreaded Dry-rot spreads unhindered over the surfaces of wooden 

 beams, in a dark, damp cellar, its mycelium presents quite a strange appearance. 

 Grouped in a circle around a dark centre of dead, disintegrating, and crumbling 

 wood are a number of white spots, joined by indistinct lines to a centre. But this 

 was not always the case. That which now forms the dead and crumbling centre 

 was formerly the seat of the first development of the mycelium, then composed of 

 a coherent network of mycelial threads and appearing to the naked eye as a single 

 rounded white spot. The mycelial threads then crept out like rays all round the 

 periphery, and as the white spot increased in diameter its centre became proportion- 

 ately dark. The mycelial threads forsook their first settlement; they died off", and 

 the wood they had destroyed then appeared merely as the dark centre of a white 

 ring. In consequence of its continual widening the ring at length becomes segre- 

 gated into stars, and is gradually transformed into a wreath of isolated mycelia, or, 

 in other words, a group of separate but distinct mycelial spots arranged in a circle 

 arises from a single mycelium in consequence of its radiating method of growth. 



The mycelium of Gasteromycetes, of many Fungi allied to Morels, and especially 

 of many Agarics growing in the forest mould or in meadow humus, also exhibit 

 under favourable conditions this ring and wreath formation. Although it is not 

 possible to see the subterranean growth directly, its results are readily recognizable, 

 since the receptacles rise above the ground from the separate portions of mycelium 

 and indicate their distribution; these receptacles occur in regular circles, and when 

 their colour contrasts with the surroundings they are especially conspicuous. 

 Rings of this kind are shown in fig. 443 formed by the Ascomycete Spatliularia 

 flavida. The subterranean mycelium of this Fungus exercises no injurious influence 

 on plants in the immediate neighbourhood — at any rate, the mosses, grasses, and 

 weeds which compose the carpet of the meadow round about show no sign of weak- 

 ness, but are equally fresh and luxuriant within and without the rings. But it is not 

 so in meadows where Agarics of the genus Marasmius and others have settled. The 

 meadow-plants whose roots and root-stocks have been penetrated by their mycelia 

 die off", and the places can be easily recognized by the withering and discoloration 

 of their green aerial parts. On first looking at these spots one might easily suppose 

 that the foundations of old circular walls were lying close under the turf which had 



