MORPHOLOGY 
185 
186 
matia that fuse with the trichogyne (fig. 185). 
Perithecia are formed, and the asci bud out from 
the auxiliary cells (fig. 186), the whole process 
suggesting the formation of cystocarps among the 
red algae. 
Lichens 
General character. With a few excep- 
tions (p. 91), lichens are Ascomycetes 
parasitic upon certain algae, the relation 
between the two organisms being so intimate 
as to result in a structure resembling a single 
organism. The dual nature of lichens was 
announced by Schwendener in 1868, but it 
was many years before the proof of it be- 
FIGS. 185, 186. stigmato- cam e convincing. In 1889 Bonnier began 
myces: 185, sex organs; right , ., . ,. , ., , . , . 
branch showing antheridia dis- tO synthesize Aliens; that IS, to bring to- 
charging spermatia; left branch gether "wild algae" and lichen fungi and 
being a procarp showing tricho- t hus produce artificial lichens. The para- 
gyne (with attached spermatia), ... . ,. . ., , ,, , , 
, .,. sitism is peculiar in that the algae do not 
carpogomum, and auxiliary 
cells; 1 86, the formation of asci seem to be harmed in most cases, the cells 
within the periihecium after being rarely penetrated by the fungus. The 
fertilization. After THAXTER. , j . ,. , , . ,. 
algae concerned in lichen formation are for 
the most part Cyanophyceae and Protococcales. As would be expected, 
the algae thrive without the fungus, just as do the wild species; but the 
lichen fungus soon perishes if it does not come into contact with the 
appropriate algae. (See p. 91 and fig. 1117.) 
Body. The lichen fungus usually forms a thallus body much more 
definite and differentiated than do other mycelia, the thallus often re- 
sembling in form that of certain liverworts. In structure, there is a 
distinct compact cortical region 
and a central looser region, in 
either of which the algae may 
occur (fig. 190). Two structural 
types of thallus body are recog- 
nized, dependent upon the dis- 
tribution of the aigae: (i) homoi- 
omerous t in which the algae are 
, i / x 7 FIG. 187. Physcia: a flat foliose lichen 
scattered ;_ and (2) heteromerous, in on bark> showing numerous shallow apothe . 
which the algae occur in layers, da. After COULTER. 
