SAPROPHYTISM AND SYMBIOSIS 331 
mutual advantage, is admirably illustrated by lichens. 
These plants grow commonly on the trunks and stems of 
trees, on old boards and fences, and on rocks (Fig. 237). 
The plant body is a thallus, and, when its inner structure 
is examined, it is seen to be a composite plant, formed by 
a species of green alga, resembling Pleurococcus, and sur- 
rounded by the mycelium of a filamentous fungus (Fig. 
238). If supplied with suitable moisture, the alga can 
live alone, because it has chlorophyll, but the fungus, 
not having chlorophyll, cannot live alone. By uniting 
into a common body, each plant supplies what the other 
needs. 
The fungal portion of lichens reproduces by means of 
spores borne in asci, and is therefore an ascomycete. The 
apothecium ("fruiting" portion of the lichen) is in reality 
a modified ascocarp (Fig. 238). In some species the 
apothecia occur at the summit of specialized, upright 
branches or podetia (Fig. 239). Only a few years ago 
the interesting discovery was made that lichens may be 
experimentally produced by the artificial union of certain 
algae and fungi (Fig. 240). Some of the lichens thus pro- 
duced resembled those found in nature, while other com- 
binations were entirely new. 
An interesting case of the symbiotic association of four 
genera, if not to their mutual benefit, at least without 
apparent detriment to either, is found in the roots of some 
of the Cycadacese. All the genera of this family produce 
alone, cannot do this. 2, Longitudinal section of apothecium; a, thecium; 
b and c, the two layers of the hypothecium; d, upper algal layer; e, colonies 
of algae distributed through the medullary layer; /, lower algal layer; g, 
lower cortical layer. 3, Cross-section of vegetative portion of thallus. 4, 
Paraphyses (sterile fungal filaments), and spore-sac (ascus), containing 
ascospores. 5, Ascospores. 6, Algal cells, surrounded by fungal hyphae 
with haustoria (absorbing branches). (After Schneider.) 
