142 



ASCOLICHENES. 



140A). The cells of the carpogonium contain one nucleus, and 

 communicate with one another by means of pits in the cell-walls. 

 The spermatia, developed in the spermogonia, become affixed to 

 the tip of the trichogyiie and empty their contents into it. After 

 this conjugation has taken place the trichogyne withers, the coiled 

 cells now divide and constitute an ascogoiie, from which the asco- 

 genous hyphge arise. The paraphyses proceed from two or three 

 cells at the base of the ascogone.] 



VEGETATIVE REPRODUCTION takes place by soredia, which to the 

 naked eye appear as whitish powder on the surface of the thallus. 

 They are small round bodies, formed by one or a group of gonidia, 

 which are surrounded by a mass of felted hyphse. After the 

 rupture of the cortex they are set free, and readily carried by the 

 wind to other places, where under favourable circumstances they 

 establish a new thallus. 



FIG. 141. A A portion of the thallus of Parmelia parietina with ascocarps (a) and sper- 

 mogonia (6). B A portion of the thallua of Celraria islandica with spermosjonia at t.e 

 end of small lobes. C A lobe with spermogonia and ejected gpermatia. (Magnified.) 



GEOGRAPHICAL DISTRIBUTION. The Lichens are the most hardy 

 plants, and are the first to appear on hitherto bare rocks which 

 they gradually disintegrate, and hence prepare the way for the 

 growth of other plants. They are to be found from the Polar 

 regions to the Equator; from the highest snow-free mountain- 

 peaks down to the level of the sea ; on the stems of trees ; on rocks, 

 soil, some even on inundated places ; on stones in woodland streams, 

 and on beaches; but they are never found upon rotten organic 

 remains. Some grow gregariously in enormous masses, and form 

 wide-stretching carpets, e.g. Reindeer Moss (Cladonia rangiferina), 

 species of Cetraria and other fruticose Lichens. 



USES. On account of the cell-wall being composed of Lichen- 



