816 THE DISTKIBUTION OF SPECIES BY OFFSHOOTS. 



round by colourless hyphal threads. They arise in the interior of the Lichen body, 

 and are ultimately pushed out on the surface by the rupture of the pustules in 

 which they arise. The wind raises and carries them away, and if they happen to 

 fall into the cleft of a rock or into the crevices on the bark of a tree they 

 immediately grow up into a new Lichen body which agrees in every particular 

 with the parent plant and is itself able to again produce soredia. The genera 

 Stereocaulon, Evernia, and Pertusaria are especially noted for their abundant 

 formation of soredia. The shrub-like, branched Stereocaulon coralloides is often so 

 thickly covered with soredia that the whole Lichen looks as if it had been strewn 

 with coarse meal, and Evernia furfuracea, growing on the bark of old trees, owes its 

 name to the fact that it seems to have been overstrewn with meal. 



It has already been stated that the multicellular offshoots (gemmae or thallidia) 

 of Mosses and Liverworts may be distributed by wind as well as by rain-water. We 

 might mention as examples, Aulacomnion androgynum, Galypogeia Trichomanes, 

 Scapania nemorosa, Jungermannia bicuspidata, and Blasia pusilla, whose offshoots 

 are borne on special erect supports (see figs. I96i5, w. i?. is, p. 23), or Syrrhopodon 

 scaher, which grows in Central America, and whose thallidia are formed on the apex 

 of the leaflets (flgs. 196 1^. is, u). The Moss TetrapUs pellucida (figs. 196 *> ^- «), which 

 grows commonly on rotten tree-trunks in Pine forests in mountainous regions, might 

 also be mentioned. It develops multicellular disc-shaped gemmae at the top of 

 certain erect shoots. The discs are supported on delicate filamentous stalks and are 

 embedded in a cup of closely crowded leaflets (figs. 196 ^' ^' '- ^). After the supporting 

 threads have withered and the small multicellular discs have become detached, a 

 slight shaking by the wind is sufiicient to make the gemmae fall out and to scatter 

 them. The .same breeze which shook the stem now whirls the tiny green discs far 

 over the forest ground and transports them to other places of attachment where 

 they continue their development. 



In some Mosses whose little leaves are very brittle when dry, for example, in 

 Gampylopus (see fig. 196^^), the leaves themselves serve as offshoots. How the 

 detachment of these leaflets is brought about is to some extent an enigma; appar- 

 ently they separate and are thrown off spontaneously, not unlike the foliage-leaves 

 which fall from the branches of trees in autumn. This is immaterial to the question 

 under consideration here, however. This much is certain, that in the remotest 

 mountain ravines, and on inaccessible ledges in precipitous places where the dis- 

 turbance of passing animals is quite impossible, the turf -forming crowded stems of 

 Gampylopus always carry detached and partially split leaflets which adhere loosely 

 to the support. When after a few dry days a storm rages through the ravines, these 

 loose leaflets are torn away, and do not again come to rest until they are far distant 

 from the spot from which they were taken. The offshoots of Mosses composed of 

 groups of cells, and the last-mentioned detached leaves which function as offshoots, 

 do not grow up immediately into new Moss-plants, but first of all develop protonema- 

 like cell-filaments, and it is from these that the young Moss-plants originate. 



It also happens that whole Moss-plants with elongated axis, numerous leaves, 



