REPRODUCTION AND DISPERSAL 807 



supply, but quite as likely because of the inhibitory effect of its own 

 excretions. Subsequently the living mycelium forms an obvious ring 

 of constantly increasing circumference, but whose thickness may not 

 increase on account of the death of the inner hyphae pari passu- with 

 the advance of those outside. Such rings are common on pots in moist 

 greenhouses. In some of the agarics, circles of toadstools, which are 

 known as fairy rings, arise from subterranean mycelial rings, producing 

 a striking effect. Sometimes fairy rings recur from a given mycelial 

 center for many years; in a colony of Hydnum suaveolens that was 

 observed for nine years, the diameter of the ring increased from seven- 

 teen to twenty-one meters, whence the age of the colony was estimated 

 at about forty-five years. Sometimes similar circles of reproductive 

 bodies, arising from rings of hidden mycelia, are associated with parasitic 

 fungi {e.g. Puccinia Pyrolae). Lichen thalli commonly spread radially 

 from the original center of establishment, the advancing edge being 

 lobate by reason of differential growth (fig. ri 1 1) ; frequently the older 

 portions die as the newer parts spread out radially, producing living 

 rings or bands, as in fungi. 



Vegetative reproduction in the hryophytes. — Thallose liverworts, such 

 as Marchantia and Riccia, spread radially from their original growth 

 center (figs. 742, 743), after the manner of lichens. As the branches 

 radiate outward and the posterior portions die, a number of individuals 

 may arise from one by isolation; it has been shown that liverwort frag- 

 ments which are only two millimeters in diameter can develop into a plant. 

 Essentially similar is the propagation of foliose liverworts and of creeping 

 mosses. Mosses increase vegetatively to a notable extent through the 

 activity of their protonema, which is composed of branched alga-like 

 filaments that creep along the soil surface much as do rhizomes. The 

 protonemal filaments bear buds that grow into leafy plants, so that the 

 area occupied by moss colonies is subject to constant radial extension. 

 Mosses possess a wonderful capacity for propagation, almost any part 

 of the leaves or stems (either gametophyte or sporophyte) being capable 

 of giving rise to protonemal filaments under suitable conditions. 



Vegetative reproduction in the vascular plants is of great significance, but has 

 been considered in connection with roots and leaves, and particularly in connection 

 with stems (p. 667). 



Gemmation. — There is another kind of reproduction which generally 

 is regarded as vegetative, although it grades insensibly into reproduction 



