72 A TEXT-BOOK OF BOTAXY. 



made up of the hyphal tissue of the fungus in which the alga is em- 

 bedded either in a single layer or throughout the mycelium. The 

 mode of growth and branching is influenced largely by the fungus, 

 although in some cases the alga may exert the most influence. In 

 some cases the lichen consists of a thallus which is irregular in 

 outline, growth taking place at no definite point, and in other 

 cases branches which are more or less regular are formed, growth 

 taking place at the apex. 



FIG. 42. Lichens showing manner of union of algae or gonidia (g) and hyphae (h) of 

 Fungi. A, Protococcus, showing the manner in which hyphae penetrate the cell and in- 

 fluence cell division; B, Scytonema, an alga surrounded by richly branching hyphae; C, chain 

 of Nostoc showing hypha of fungus penetrating a large cell known as a heterocyst; D, fungal 

 hyphae have penetrated the cells of Glceocapsa, a blue-green, unicellular alga; E, Chlorococcum, 

 a reddish or yellowish alga found in Cladonia furcata, the cells of which are surrounded by 

 the short hyphae of the fungus. A, after Hedlund; B-E, after Bornet. 



The walls of the hyphae of the fungus comprising Lichens con- 

 sist at first of pure cellulose. In older material the walls undergo 

 more or less modification, being changed in part to starch, mucilage, 

 or fixed oil. There may be also infiltrated among the layers of 

 the wall calcium oxalate, the latter constituent being especially 

 characteristic of the crustaceous Lichens. The most interesting 

 constituents of Lichens are the coloring principles, which are 

 mostly of an acid character and are termed Lichen-acids. They 



