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DIVISION III. — MODE OF LIFE OF THE FUNGI. 



therefore be separated from it without much injury ; and the crus/accous (thallus 

 crustaceus, lepodes), a flat crust on or in the substratum and adhering firmly to it 

 at least by its whole under surface, so that it cannot be separated from it without 

 injury. The genera Cladonia and Stereocaulon are peculiar, having shrub-like 

 formations {podetid) rising from scaly or granular foliaceous bodies (the thallus or 

 protothallus of the Lichenologists). These forms all agree in two points of structure ; 

 they are composed, as has been said, of hyphae and of algal cells inserted between 

 them, and from the first commencement of their formation hyphal branches from the 

 base or surface turned towards the substratum enter the subtratum as rhizoid-hyphae, 

 which sometimes, especially in foliaceous species, are united into rhizoid-strands. 

 These organs by which the plant attaches itself to the substratum and takes up food- 

 material arc termed by Lichenologists 

 rhizincs, and they often penetrate, 

 especially in species living on rock or 

 soil, as much as a centimetre deep into 

 the substratum and spread through it in 

 dense ramifications. The rhizoids of 

 thespecies which live in the rind of trees 

 seem never to penetrate far into it, at 

 all events they do not reach the living 

 tissue of the rind. In many species, 

 which, like the Graphideae examined by 

 Frank and Lecanora pallida, form thin 

 crusts in the rind of trees, we find in 

 place of distinct rhizoid-branches the 

 system of hyphae described above, which 

 spreads in superficial peridermdayers 

 without going further inwards. These 

 hyphae may form a persistent thallus 

 which grows all its life entirely in the 

 periderm along with the Algae which it 

 has attacked, as in the above-mentioned 

 and similar Graphideae and in Pyrenula 

 nitida, and it may be covered by one or 

 more layers of periderm-cells, in which 

 case it is termed by Lichenologists hypophloeodic ; or it makes its way out to the outer 

 surface of the periderm in consequence of the subsequent growth in thickness of 

 the parts which contain the Algae, and forms there an epiphloeodic crust. 



In other points of structure differences appear, which have no simple relation 

 to these growth-forms, but may be repeated in each of them. Lichenology since 

 Wallroth's time has accordingly distinguished between the hctcrojnerons and the 

 homotomerous thallus. The former is peculiar to the 'true Lichens' (Lichenes of 

 Fries, Lichcnaceae of Nylander, Gnesiolichenes of Massalongo), the latter to the Phyco- 

 lichenes of Massalongo (Collemaceac of Nylander, Byssaceae of Fries). The hetero- 

 merous thallus occurs in the large majority of species, and displays in fact a structure 

 the main features of which can be clearly defined. The homoiomerous on the contrary 



M 



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Fig. 171. Usnen barbata. A optical longitudinal section of 



the extremity of a thin branch of the thallus which has 1 v 



transparent in solution of potash (see Fig. 86 A). B tran 

 section through a stronger branch with the point of origin of an 

 adventitious branch sa; r cortical layer, m medullary layer, \ 1 



a*xile strand, g the algal zone {Cystococcus), s apex of the branch. 

 After Sachs. Magn. nearly 100 times. 



