TYPES OF CRYPTOGAMS; THALLOPHYTES 273 
Examine the spore-sacs (asci) and look for spores in different stages 
of formation. How many spores are found in each ascus ? What other 
bodies occur among the asci? Draw these, also asci and spores. 
331. Lichens.— Lichens were formerly supposed to be 
a distinct class of plants, and it is only about thirty years 
since their real nature began to be understood. A lichen 
is now known to be a combination of two plants. The 
green cells, called the gonidia, belong to some species of 
alga, and the remainder, the larger portion of the growth, 
is a fungus parasitic upon that alga. The groups of 
lichens correspond in structure to certain groups of fungi, 
but the genera are sufficiently distinct so that lichens are 
best considered by themselves for purposes of study and 
classification. 
The relation of the fungus and its algal host is not 
that of destructive parasitism, but rather a mutual rela- 
tion (symbiosis) in which both fungus and alga may have 
a vigorous growth. The relationship has been investi- 
gated in various ways, and it has been found that, while 
the alga may grow independent of the fungus, the germi- 
nating fungus spores can grow only to a limited extent if 
deprived of the algal host; but if supplied naturally or 
artificially with the proper alga they make a normal 
growth. 
The same alga may serve as gonidia to a number of 
lichens, often of very different form, and while the num- 
ber of lichens reaches into the thousands, the number of 
algz known to serve as gonidia is quite small. 
Lichens are widely distributed in all zones but flourish 
particularly in northern regions where other vegetation is 
scanty. Some were formerly important as sources of 
