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A TEXTBOOK OF BOTANY 



[Ch. X 



suggests the nature of the relation between the two, viz., 

 the Fungus is parasitic upon the Alga, though the parasitism 

 must be of a very mild sort, since the Alga suffers little if 

 any injury therefrom. Indeed, it is possible that the Fungus 

 is simply saprophytic upon the photosynthate the Alga 

 produces. The Algae, for their part, derive a certain minor 

 benefit from the association, because the translucent thallus 

 covers and protects them from desiccation and excessive 



Fig. 323. — Fruticose Lichens, Cetraria islandica, x \ (left), and Cladonia 

 rangiferina, X \. (From Figurier, and Warming.) 



insolation, thus permitting them to grow in much drier and 

 more exposed places than they could otherwise endure. 

 The relation between the two has been compared with that 

 of master to slave, and on that account is sometimes called 

 helotism. It is of course because the Fungus does no serious 

 damage to the Alga that they are enabled to grow perennially 

 together. 



The reproduction of the Tree Lichen, through ascospores 

 formed in an apothecium as in Discomycetes, is typical of 

 most Lichens, but some have perithecia. While fertilization 

 precedes the formation of the ascocarp in many species, in 

 others it does not, the sexual mechanism being more or less 

 eliminated. In addition, many kinds reproduce vegeta- 

 tively, either by portions of the thallus incidentally sepa- 

 rated, or else from regularly-formed soredia, i.e. rounded 



