CHAPTER VII.—PHENOMENA OF VEGETATION.—LICHENS. 411 
Graphis (Opegrapha) varia, O. plocina, Kbr., O. saxatilis, Schaer. and many other 
species, together with Lecanactis illecebrosa, Kbr., species of Arthonia, and Pyrenula 
nitida, in which the crustaceous thallus with progressive marginal growth consists of 
an air-containing weft of slender hyphae, which grow round filaments of Chroolepus 
as their Alga without any differentiation into medulla and rind (see above on p. 399). 
For further details in these cases which require more exact investigation the reader is 
referred to special treatises. 
3. Another special form of structure must now be mentioned, the granular 
green thallus of certain Lichens living on the ground which Stahl has examined 
carefully in Thelidium (Fig. 175). In this case the thallus is formed of a loosely 
woven mass of hyphae which vegetate beneath the surface of the ground. Branches 
of these hyphae rise to the surface and attach themselves as in germination to the 
proper algal cells there present; the particular Alga of Thelidium is Pleuro- 
coccus. The hyphal branches grow completely round the cells forming a thin layer, 
and so follow the growth of the Alga that this relation between the two elements is 
constantly maintained, the hyphae pushing in between the products of the division of 
the algal cells as they are formed one after another, and twining round them as at the 
first. In this way groups of Algae are formed lying on the hyphal thallus and 
surrounded and held fast by the hyphae. No further differentiation takes place in 
them. It is possible that the process is the same in Biatora vernalis and similar forms, 
which should be examined on this point. 
4. The thallus in Chiodecton nigrocinetum, Mont., Byssocaulon niveum, 
Mont. and Coenogonium Linkii (Fig. 176 e, f) consists of a branched filamentous 
Alga, belonging according to Bornet to the genus Trentepohlia and probably the 
same as Kiitzing’s Chroolepus flavum, and surrounded by a dense weft of fungal 
hyphae; Coenogonium confervoides, Nyl. consists of a larger species nearly 
approaching according to Bornet to Chroolepus villosum and wrapped in a similar 
fungal weft. In these forms of thallus the Alga either does not alter its growth at all, 
or very little. In Coenogonium the Lichen has the conferva-form and the Alga advances 
by longitudinal growth, so that the tips of its branches are often found free from the 
hyphae. A similar structure is seen in the European species Racodium rupestre, Fr. 
(Cystocoleus of Thwaites) which is known only in the sterile state, with the difference 
however that here the hyphae which acquire thick and brown membranes surround 
the apex also of the branches of the Chroolepus in a single layer which has no inter- 
stices (Fig. 176, a-d). We learn from Bornet that Opegrapha filicina, Mont. behaves 
in the same way as Coenogonium, only that there the Alga is one of the species 
of Phyllactidium which form scutiform cell-layers on the leaves of tropical Ferns. 
5. The Collemaceae or gelatinous Lichens are a group in which the 
Algae are Nostocaceae or Chroococcaceae with gelatinous membranes or membrane- 
sheaths; the Fungus makes its way into the gelatinous membrane, and, with the 
exception of the rhizoids and the sporocarps which issue from it, grows entirely in- 
side it and ramifies in it. Fungus and Alga are differently related to one another in 
the common growth in the different species, but the Fungus never dominates the Alga 
so decidedly as in the heteromerous forms. The form and structure of the Alga is 
accordingly different in different cases, but the structure always varies less than the form. 
