186 STUDIES IN CRYPTOGAMS 



LICHENS 



Lichens are so common everywhere that the attention of the 

 student is sure to be drawn to them. They grow on rocks (Fig. 34G), 

 trunks of trees, old fences, and on the earth. They are too difficult 

 for beginners, but a few words of explanation may be useful. 



Lichens were formerly supposed to be a distinct or separate tribe 

 of plants, and many species have been described. They are now known 

 to be the green cells of various species of algae, overgrown and held 

 together (imprisoned) by the mycelium of various kinds of fungi. 

 The result is a growth unlike either component. This association of 

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

 ful growth, the alga furnishing some things, the fungus others, and 

 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. 

 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 fungous threads. These are readily 

 observed in many lichens. They also produce spores, usually asco- 

 spores, 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 hyphse 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. 331, 332. The plant consists of a flat ribbon- 

 like thallus which creeps along 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 



