ANATOMY OF TISSUES. 79 
name, though it was given them before the true uature of their 
relation to the hyphae threads was known. The hyphae com- 
posing pith and rind are not at all different from those of other 
fungi; their walls consist of fungous cellulose. 
The algae which are found in the lichens are similar to those 
which exist as separate plants ; they occur either in chains or 
as single cells. They contain chlorophyll, and in some instances 
FIG. 38. 
Tissue of Rocella tinctora, teased out with needles to show the form of the elements. 
Jf threads of the felt-like tissue. o uninjured, o’ corroded crystals of calcium 
oxalate. g gonidia. x 300.—(Wiesner.) 
have been freed from their fungous companions, and found to 
erow on as independent algae. 
The most common form of: lichen is a thallus, resembling 
somewhat that of the Hepaticae in the next higher group. To 
this there is no exact homologue in the real fungi. It has been 
suggested that the sclerotium formed by many of the higher 
fungi may be considered the corresponding organ. Even ad- 
mitting this to be true, the mycelium form of the true fungus 
is wanting in the lichen. 
The three foregoing classes, fungi, algae, and lichens, are 
generally considered true thallophytes, or plants with no dis- 
tinction of stem and leaf. Among the algae, however, are 
