404 



DIVISION III. MODE OF LIFE OF THE FUNGI. 



medullare). Both layers belong to the hyphal system, their constituents are ramifi- 

 cations of the same hyphae. The cells of the Alga are in almost all cases inserted 

 where the medulla and the rind meet. Together they form a green zone of varying size 

 projecting into the medullary tissue to a different depth at different points, everywhere 

 traversed by single hyphae of the medulla running to the rind, and in some places 

 showing larger interruptions. This is the algal layer or algal zone which is commonly 

 termed the third tissue-layer. But single algal cells or groups of cells are often found 

 scattered through the medulla, as in Solorina and Placodium, or the whole mass of 

 algal cells is distributed with tolerable uniformity through the medulla, as in Bryopogon. 

 Sundry modifications of this type and small deviations from it occur in different 

 species and groups of species. The following are the most important ones : 



Some fruticose thalli, especially if cylindrical In form, are covered all round with 



a uniform rind, as in Usnea (Fig. 171), 

 Bryopogon, Roccella, Sphaerophoron 

 and others. In many fruticose forms 

 with a flat thallus (species of Evernia 

 and Cetraria) and in most foliaceous 

 forms the rind of the upper surface 

 which is towards the light is different 

 from that of the lower surface (Figs. 

 I 7 2 > 173) ; in Anaptychia, Peltigera, 

 Solorina and most of the foliaceous 

 portions of the thallus in the Cladonieae 

 the upper side only has the rind up to 

 the margin, the lower surface has no 

 rind. 



When there is this difference in the 

 two surfaces, the algal zone apart from 

 the scattered cells only appears on the 

 side towards the light. Even in the 

 almost cylindrical thallus of Sphaero- 

 phoron with a uniform rind all round it, it 

 is often more strongly developed on the 

 side towards the light than on the lower 

 one. The podetia of Cladonia, which 



according to Krabbe ' are to be regarded as strongly developed parts (envelope-appara- 

 tus) of apothecia but which always have the structure and growth of a fruticose thallus, 

 are always without a rind when young, and in some species, as C. rangiferina, never 

 have a rind, their surface being formed of a loose hyphal weft containing Algae. Most 

 species are at a later period of their life partially covered with small wart-like rind-scales, 

 and some are completely covered with a rind, as C. furcata. 



Some of the crustaceous forms, for example the genera Psora, Psoroma, Placodium, 

 Endopyrenium &c. and Thalloidima candidum are only distinguished from the folia- 

 ceous by having their under surface covered all over with rhizoid-hairs which penetrate 

 into the substratum. These hairs either spring as branches from the elements of a 

 distinct rind, as in species of Placodium and Endopyrenium, or where the rind 

 is wanting on the under surface they are direct continuations of the medullary hyphae. 



Other crustaceous Lichens, those namely with areolate surfaces or which have a 

 granular appearance, have essentially the same structure in the fully developed areolae 

 as the species mentioned above which are without a rind on the under surface. Some of 





FlG. 174. LtcitUlla cnterolcuca, Kbr. on the rind of Tilt 

 surface-view of the margin of a young growing thallus. Jj 

 vertical longitudinal section through the same, m black marginal 

 bands, p groups of Algae, /periderm. A slight'y magnified. Circum- 

 ference of B magn. 45 times, but the whole completed from greater 

 enlargement. 



A 

 dW 



1 See on page 222. 



