268 LICHEN ES. 



being set free on the disintegration of the crust. Recent 

 observations however show that the " gonidia " alone 

 cannot reproduce the Lichen, but merely their like ; that 

 they give origin to other similar cells, but nothing more, 

 and that they present precisely the characters of the 

 lower groups of Algas, to various genera of which Order 

 they may be actually referred. Intermingled with the 

 germ-cells and in intimate contact with them, as well as 

 usually forming alone the stratum of the crust imme- 

 diately beneath that in which the gonidia abound, are 

 long irregularly ramified thread-like cells, destitute of 

 chlorophyll, matted often densely together, forming the 

 mass of the crust. From the cortical cells of the lower 

 surface copious root-like hairs are given off which serve 

 as holdfasts to the thallus. 



Upon the upper surface of the crust and usually visible 

 to the naked eye, occurs the so-called "fructification," 

 although of asexual origin, in the form of convex, flat, or 

 saucer-like disks in some Lichens (Gymnocarpous or 

 disciferous genera), or in others as rounded cavities em- 

 bedded in the substance of the crust, opening at the 

 surface merely by a minute aperture (Angiocarpous or 

 nucleiferoits genera). These disks or cavities are lined 

 closely with erect, narrow, often clavate, densely packed, 

 microscopic cells, amongst which are numerous asci, each 

 containing usually eight or fewer spores, originating, 

 as in ascomycetous Fungi, by free-cell formation. These 

 spore-containing asci originate from, and in direct con- 

 tinuity with, the colourless filaments which form the mass 

 of the Lichen, and are not in any organic relation with 

 the gonidia. 



The spores are liberated in damp weather, and on germi- 

 nation develop thread-like filaments similar to those 

 which, matted together, form the tissue from which the 

 asci originate. 



It follows therefore that the Lichen may be regarded- 

 as a composite structure, made up of Alga and ascomy- 

 cetous Fungus, each element capable of at least inde- 

 pendent asexual propagation, but incapable alone of< 

 reproducing a Lichen which necessarily requires the 

 concourse of both. Since, however, Lichens thus con- 

 stituted occupy a very important position in the Vegetable 



