196 PLANT BIOLOGY 



alga and fungus is usually spoken of as symbiosis, or mutually 

 helpful growth, the alga furnishing some things, the fungus others, 

 and both together being able to accomplish work that neither 

 could do independently. By others this union is considered co 

 be a mild form of parasitism, in which the fungus profits at the 

 expense of the alga. As favorable to this view, the facts are cited 

 that each component is able to grow independently, and that under 

 such conditions the algal cells seem to thrive better than when 

 imprisoned by the fungus. 



Lichens propagate by means of soredia, which are tiny parts 

 separated from the body of the thallus, and consisting of one or 

 more algal cells overgrown with fungus threads. These are readily 

 observed in many lichens. They also produce spores, usually 

 ascospores, which are always the product of the fungus element, 

 and which reproduce the lichen by germinating in the presence of 

 algal cells, to which the hyphae immediately cling. 



Lichens are found in the most inhospitable places, and, by 

 means of acids which they secrete, they attack and slowly disin- 

 tegrate even the hardest rocks. By making thin sections of the 

 thallus with a sharp razor and examining under the compound 

 microscope, it is easy to distinguish the two components in many 

 lichens. 



LIVERWORTS 



The liverworts are peculiar flat green plants usually found 

 on wet cliffs and in other moist, shady places. They frequently 

 occur in greenhouses where the soil is kept constantly wet. 



FIG. 288. FIG. 289. 



PLANTS OF MARCHANTIA. 



One of the commonest liverworts is Marchantia polymorpha, 

 two plants of which are shown in Figs. 288, 289. The plant 

 consists of a ribbon-like thallus that creeps along the ground, 

 becoming repeatedly forked as it grows. The end of each branch 



