LICHENS. 305 



the term hypothallus is applied, and which constitutes the 

 vegetative system of the future lichen. So far the develop- 

 ment is the same as that of the fungi ; but at a longer or 

 shorter period after the formation of the hypothallus we 

 may observe upon its surface a whitish layer of spheroidal 

 cellules, intimately united with each other as well as with 

 the filaments from which they take their origin. This 

 layer is the groundwork for a second formation of globular 

 cells, and these are only to be distinguished from the 

 first by the chlorophyll which they contain. They are 

 called gonidia, and are peculiar to Lichens. Such is the 

 formation of the most simply organized of the class, as the 

 Verrucarice, the receptacles (apothecia) of which closely 

 resemble those of a Sphcerla, and are found upon the sur- 

 face of the hypothallus. In the more complicated foliaceom 

 Lichens, as Parmelia, the mature thallus is made up of two 

 kinds of tissues, the medullary and the cortical. The cor- 

 ticular portion forms the layers, an inferior and superior, 

 and consists of thick- walled cells, closely adherent to 

 each other ; from the surface of the inferior layer are given 

 off numerous root-like appendages, on either side of which, 

 or rather embedded in its cortical substance, are the gonidia, 

 which form a green tissue. Of the spore-like organs, 

 spermatia and stylospores, there are three varieties, to 

 which the terms apothecia, spermogonia, and pyenides 

 have been applied. The most common form of the apo- 

 thecium is that of the disc, which may be plane, convex, 

 or cup-shaped. This form is that which characterises the 

 Gymnocarpous Lichens. In the Angiocarpece the organ is 

 closed upwards, its superior surface becoming internal, so 

 as to form a conceptacle like that of the Pyrenomy- 

 cetes; the form, however, of which is subject to much 

 variation. 



The reproductive organs of Lichens, as in Fungi, are of 

 five kinds . — 1, Sporules, which are formed by the con- 

 struction and subsequent separation of the extremity of a 

 simple cylindrical filament ; 2, Spermatia with their sup- 

 porting pedicles ; 3, Stylospores with their styles ; 4, 

 Thecae or asci ; 5, Basidia with their basidiospores. As 

 regards the complexity of their form and structure they 

 may be taken in the order in which they are here placed ; 



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