350 FLORIDE/E I CORALLINES. LICHENS. 



set are probably gemmce, whilst the other are germ-cells ; but it 

 is not yet determined to which of the two these characters respec- 

 tively belong. The ' Tetraspores ' — which are peculiarly charac- 

 teristic of this group, being found in every one of its subdivisions 

 — are usually imbedded in the general substance of the frond, 

 though they sometimes congregate in particular parts, or are 

 restricted to a special branch. Each group (Fig. 164, b) seems to 

 be evolved within one of the ordinary cells of the frond, which 

 undergoes binary subdivision ; the four secondary cells, however, 

 remain enclosed within their primary cell until the period of 

 maturity, a new envelope, the 'perispore,' being formed around 

 them. — In the true Corallines, which are Sea-weeds whose tissue 

 is consolidated by calcareous deposit, the Tetraspores are included 

 within hollow conceptacles ; but, generally speaking, it is the 

 simple Spores only which are thus specially 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 Gene- 

 rative spores, whilst the Tetraspores are Gemma?, as Harvey 

 and Thwaites consider them ; but a different view is taken by 

 Decaisne, Agardh, and other eminent Algologists, who regard the 

 tetraspores as the true generative spores, and consider the simple 

 spores to be gemma?. 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. 



261. The Class of Lichens, which consists of Plants that closely 

 correspond with Algse 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 examination 

 of it more than ordinarily difficult. Lichens are commonly 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 exposed to light and air. 

 In the simpler forms of this group, the Primordial Cell gives 

 origin, by the ordinary process of subdivision, 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 defined 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 



