DISPERSAL BY WIND. 



813 



distributed in small detachments, and only at a suitable time, i.e. when a dry wind 

 IS blowing. A similar contrivance is exhibited by the Muscinese in the Mar- 

 chantiaceae, Anthocerotaceas, and Jungermanniacese. Peculiar filamentous, very 

 hygroscopic cells with spiral bands of thickening on the cell-wall, are found with 

 the spores in the receptacles of these plants (see p. 696). These have been called 

 elaters, because it was thought that their movements caused the ejection of the 

 spores. Their significance, however, rather lies in the fact that they serve to hold 

 the spores together after the opening of the receptacle, and only expose them bj 

 degrees to the wind. They also help to burst open the receptacles, but that hardly 

 concerns us just now. 



Only three of the most striking of the 

 varied contrivances for spore distribution by 

 wind in Mosses (which are destitute of elaters) 

 will be here described. First, those which are 

 observed in the Andreseaceee (see figs. 450 ^ and 

 450 ^). Here the capsule opens by four longi- 

 tudinal clefts which, however, do not extend 

 quite to the free end, and the four pieces into 

 which the wall is thus divided may be com- 

 pared to the staves of a barrel joined together 

 at the top. In damp weather they become 

 approximated, so that the clefts are closed (fig. 

 450^). In dry weather the valves become 

 arched, the clefts widen, and the spores may be 

 blown out from the interior of the capsule by 

 the dry wind (fig. 450 ^). The distribution of 

 the spores is effected quite differently in the 

 Polytrichums, one species of which is illustrated 

 in figs. 450 ^■*'^'^''^'^. After the roof (operculum) which formerly surmounted the cap- 

 sule has fallen off a delicate whitish membrane comes into view, which is held fast by 

 the points of numerous sharp teeth, and is stretched like the skin of a drum over 

 the mouth of the capsule with its annulus. If rain and dew moisten the Moss the 

 teeth are seen to be much bent inwards, the membrane lying upon the annulus, and 

 completely closing the receptacle (fig. 450 ^ and 450 ''). But in dry air, especially 

 when a dry wind is blowing, the teeth turn rather outwards, raising the membrane 

 above the annulus, and thus small holes are left between the teeth through which 

 the spores can escape (figs. 450 ^ and 450 *). The same dry wind which causes the 

 alteration in the position of the teeth now shakes the spores out of the capsule, 

 which is borne on an elastic seta. Grimmia ovata, one of the Bryaceae (see 

 figs. 450^ and 450^"), may be taken as the type of a third contrivance for ex- 

 posing the matured spores to the wind in dry weather, retaining them in the 

 receptacle when it is damp and protecting them there from the injurious effects of 

 moisture. The circular mouth of the pipe-bowl-shaped receptacle is furnished with 



449.— rWcAm clavata. 



1 The membrane of the sporangium has burst, and 

 the capillitium has bulged out raising up the 

 spores embedded between its threads and expos- 

 ing them to the wind ; x 20. 2 Threads of tlie 

 capillitium with the spores lying between them; 

 x260. 



