532 



HUTCHINSON'S POPULAR BOTANY 



Species that perform the latter operation are classed as parasites; those 

 that content themselves with organic remains are saprophytes. Members 

 of either class that have the power during the whole or part of their 

 life of performing the functions of the other class are qualified as facul- 

 tative saprophytes or parasites, as the case may be. Another group live 

 symbiotically with algse, and are known as lichen-forming fungi Lichens, 

 formerly considered as a distinct subdivision of Cryptogams, being now 

 known as compounds of fungal and algal elements (figs. 671-675). 



As the Fungus is obviously the dominant partner in this joint-stock 

 company, it is usual to treat of Lichens under the head of Fungi, but from 

 the field naturalist's point of view it is still convenient to consider them 

 as though they were simple organisms, and to retain their distinctive names. 



Many of them are among the 

 most beautiful of our crypto- 

 gamic plants, and adorn what 

 they grow upon, whether it 

 be an old wall, an alpine 

 rock, the trunk or branch of 

 a tree, or the bare earth of 

 moor or woodland. These 

 differences of situation 

 adopted by the various so- 

 called species produce great 

 variety of form and habit, 

 some sitting so closely that 

 they might be regarded as 

 mere stains or dabs of paint. 

 Others spread out flat 

 branching lobes or hang as 

 grey beards from trees, and 

 some spread over the ground 

 or grow up from it like miniature shrubs. Among the latter are the 

 Reindeer Moss (Cladonia rangiferina, fig. 104), and the pretty Cup Mosses 

 of the same genus (fig. 674). The Dog Lichen (Peltigera canina, fig. 677) 

 spreads along the ground, forming patches five or six inches in diameter, 

 much like a large Liverwort, but of a more leathery texture. Its names are 

 accounted for by the fact that in former days it was considered a specific 

 for hydrophobia. This belief may have been due, under the " Doctrine of 

 Signatures," to the dog-tooth-like appearance of the fruits along the margin 

 of the lobes. The herbalists called it Ground Liverwort, under the im- 

 pression that it was a kind of Marchantia. A similar " signature " caused 

 the prescription of the Lichen known as Lungs of Oak (Sticta pulmonaria) 

 in pulmonary troubles, the pitted underside being supposed to resemble the 

 structure of the lungs and so to indicate its suitability for mending them 



FIG. 676. POTATO-BLIGHT (Phytophthora infestans). 



One of the Phycomyceteae. At (a) the hypha is seen growing out through 

 a stomate of Potato leaf and bearing sporanges. (6) Zoospores. (c) A 

 germinating zoospore. 



