BOTANY. 61 



perpendicularly to the outer layer. The fructification gradually projects 

 more and more from the surface, and either remains covered with the outer 

 layer, or bursts through it. When it remains closed, there is a nucleus in 

 the centre. When the fructification bursts through the cortical or outer 

 layer, it expands in the form of shield-like disks, called apothecia or patellae, 

 or linear expansions called lirelhis. Sometimes the cortical matter forms 

 a border round the fructification, aj; other times it grows up in the form of 

 a stalk, so as to give rise to the podetium. The young thecas (asci) contain 

 spores, varying from four to eight, or from twelve to sixteen. Occasion- 

 ally, the spores are in sets of two. Separated cells of the medullary layer, 

 of a green color, called gonidia, or gongyli, are considered as another kind 

 of reproductive organ. There is much uncertainty as to the real character 

 of the spherical or sub-spherical green bodies called gonidia, which are 

 characteristic of true lichens. When separated from the parent structure, 

 they are capable of forming new plants. Lichens are found in all quarters 

 of the globe, adhering to stones, rocks, trees, «fee. During their entire 

 growth, they appear to be capable of deriving most of their nourishment from 

 the atmosphere. They have the power of acting on hard rocks, so as to dis- 

 integrate them in process of time, and many of them contain much inorganic 

 matter in their composition. They all grow in the air ; none are found sub- 

 mersed. 



Sub-order 1. CojiiothalamecB : pulverulent lichens ; shields open, without a 

 nucleus, cavity filled with free spores. 



Suh-order 2. Idiothalamea. : shields closed at first, opening afterwards, con- 

 taining free spores in a nucleus composed of the gelatinous remains of the 

 paraphytes and sporangia. 



Sub-order 'i. G aster othalamea. : shields either closed always, or opening 

 by bursting through the cortical layer of the thallus, the nucleus containing 

 the deliquescing or shrivelled sporangia. 



Sub-order 4. HymenothalamefB : shields open, discoid permanent, nucleus 

 bearing the sporangia on its surface. 



The economical value of some lichens is considerable. Cetraria islandica 

 {pi. 54:, ßg. 31), or lijeland Moss, contains a nutritious substance called Hch- 

 enin. Cladonia rangiferina furnishes the principal winter food of the Reindeer. 

 Fig. 29 represents Cladonia pyxidata and verticillata. Parmelia parietina 

 {pi. 54, ßg. 30) contains a yellow coloring matter called parietin. Rocella 

 tinctoria {ßg. 32) furnishes part of the archil of commerce. Fig. 33 repre- 

 sents Usnea florida. The tripe de roche, a nutritious lichen found in the 

 Arctic regions of America, belongs to the genus Gyrophora. 



Sub-class 2. ^tlieogamcc or Cormogence. 



Order 4. Hepatic^e, the Liverwort Family, Plants having an axis 

 which either bears cellular leaves or is leafless, and is bordered by a mem- 

 branous expansion or thallus. Stomata are found in the epidermis of 

 some. The reproductive organs are : 1. Antheridia, which are either 



61 



