STRUCTURE AND REPRODUCTION OF LICHENS. 333 



protected. These are never scattered through the frond, like the 

 tetraspores ; and are commonly developed within a ceramidium, 

 which is an urn-shaped case, furnished with a pore at its summit, 

 and containing a tuft of pear-shaped spores arising from the base 

 of its cavity. The resemblance of these bodies in position to the 

 " octospores" of Fuci, would seem to justify the conclusion that 

 they are the true generative spores, whilst the tetraspores are 

 gemmae, as Harvey and Thwaites consider them ; but a different 

 view is taken by Decaisne, Agardh, and other eminent Algolo- 

 gists, who regard the tetraspores as the true generative spores, 

 and consider the simple spores to be gemmae. It is, therefore, a 

 point of much interest to determine by careful observation and 

 experiment which is the right view; and Microscopists who have 

 the opportunity of studying these plants, either in their native 

 haunts, or in artificial Aquaria, can scarcely apply themselves to 

 a better subject of investigation. 



207. The class of Lichens, which consists of plants that closely 

 correspond with Algae in simplicity of organization, but differ 

 from them widely in habit, does not present so many objects of 

 attractive interest to the Microscopist ; and the peculiar density 

 which usually characterizes their structure, renders a minute ex- 

 amination of it more than ordinarily difficult. Lichens are com- 

 monly found growing upon the trunks or branches of trees, upon 

 rocks or stones, upon hard earth, or in other situations in which 

 they are sparingly supplied with moisture, but are freely ex- 

 posed to light and air. In the simpler forms of this group, the 

 primordial cell gives origin, by the ordinary process of subdivi- 

 sion, to a single layer of cells, which may spread itself over the 

 surface to which it is attached, in a more or less circular form ; 

 and one or more additional layers being afterwards developed 

 upon its free surface, a thallus is formed, which has no very de- 

 fined limit, and which, in consequence of the very slight adhesion 

 of its component cells, is said to be " pulverulent." Sometimes,, 

 however, the cells of the thallus are rather arranged in the form 

 of filaments, which penetrate the superficial layers of the bark 

 whereon such Lichens grow, and which are sometimes also so 

 interwoven at the outer surface, as to form a sort of cuticle. 

 Interposed among the ordinary cells of the thallus, we very com- 

 monly find certain green globular cells, arranged in single bead- 

 like filaments ; these, which are termed gonidia, being found to 

 be capable of reproducing the plant when detached, must be 

 considered as gemmce. In the higher tribes of Lichens, we find 

 the interlacing filaments forming a tough cortical envelope to 

 both surfaces; whilst in the interior of the firm " crustaceous" 

 thallus, the gonidial cells are found in regular layers. Some- 

 times these increase in particular spots, and make their way 

 through the upper cortical layer, so as to appear on the surface 

 as little masses of dust, which are called soredia. Besides these, 

 Lichens contain proper Generative organs, by which a true sexual 



