194 
fungal symbiont.. Three types of jichen thallus may be recognized, 
namely, the crustaceous, the foliaceous and the fruticulose. The 
crustaceous type is the most rudimentary and cannot be said to 
to have even a dorsiventral structure, though one would naturally 
expect this from the nature of things. The lower surface differs 
only in having more numerous extended hyphal filaments to en- 
able it to adhere more firmly to the substratum as well as to take 
up soluble food materials. The second type already indicates a 
considerable advance in the evolution of the lichen thallus. It is 
typically dorsiventral. Dorsal and ventral layers are semi-cortical 
in structure ; that is, the hyphal cells are closely united and have 
only few air passages. Between these two layers is a layer 
of loosely interwoven hyphal tissue in which are imbedded the 
algae. From the lower surface extend the rhizoids. On the 
_ upper are found the apothecia (with exceptions, example, Vepro- 
muum) and soredia, besides the so-called “ spermagonia” and oc- 
casionally accidental fungal and algal parasites. The third type 
(as exemplified by the vertical thallus of Cladonia and Zhamnolia) 
shows a typical radial structure. Numerous examples showing 
the gradual gradation from the dorsiventral to the radial type can 
be found. In the radial type there is an outer semi-cortical layer, 
which usually differs from that of the dorsiventral cortical layer 
in that itis more compact. The fungal cell walls have become 
somewhat gelatinized and adhere very closely. Next to this layer, 
on the inside, is the layer of loosely interwoven hyphae containing ~ 
the algae. The third and innermost layer consists of longitu- 
dinal closely united hyphae. Sometimes this thallus is hollow in 
the center, sometimes solid, containing a central core of closely 
united longitudinal hyphae. 
Soredia are also typical lichen structures. They are very 
numerous in the higher forms of lichens (example, Parmelia sore- 
diata), and are found on the dorsal surface of the thallus, more 
frequently near the margin. Each soredium isin reality a miniature 
thallus. It is usually spherical in form, the outer layer consisting 
_ of closely united hyphal cells; the central portion consists of algal 
cells and loosely interwoven hyphal filaments. Soredia contain 
all the elements necessary for the development of a new lichen. 
From what has been stated above, apothecia can not be looked 
