LICHENS AND HEPATICS 193 



LICHENS 



Lichens are so common everywhere that the attention of the student 

 is sure to be drawn to them. They grow on rocks (Fig. 373), trunks 

 of trees, old fences and on the earth. They are too difficult for begin- 

 ners, but a few words of explanation may be useful. 



Lichens were formerly supposed to be a distinct or separate divi- 

 sion of plants. They are now known to be organisms, each species of 

 which is a constant association of a fungus and an alga. The thallus 

 is ordinarily made up of fungous mycelium or tissue, within which the 

 imprisoned alga is definitely distributed. This association of alga and 

 fungus is usually spoken of as symbiosis, or mutually helpful growth, 

 both together being able to accomplish work which neither could do 

 independently. By others this union is considered to be a mild form 

 of parasitism, in which the fungus profits at the expense of the alga. 

 Each component may be able to grow independently, and 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 sepa- 

 rated from the body of the thallus, and consisting of one or more algal 

 cells overgrown with fungous threads. These are readily observed 

 in many lichens. They also produce spores, usually ascospores, which 

 are always the product of the fungous 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 disintegrate 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 grow- 

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

 occur in greenhouses where the soil is kept constantly wet. One of 

 the commonest liverworts is Marchantia polymorpha, two plants of 

 which are shown in Figs. 358, 359. The plant consists of a flat ribbon- 

 like thallus which spreads over the soil, becoming repeatedly forked 

 as it grows. The end of each branch is always conspicuously notched. 

 There is a prominent midrib extending along the center of each branch 



