214 PHYLUM VII. CARPOMYCETEAE 
Tue Disk ‘‘Licuens’” (ORDER DISCOLICHENES) 
337. The primitive Ascus Fungi (Ascosporeae) appear to 
have been parasitic on small, green algae (MyxXOPHYCEAE 
and KHLOROPHYCEAE), and indeed this may have first 
taken place in the water. It is known that some of the 
proper Red Algae are parasitic, and the view here taken is 
thatin the Disk Lichens we have a group of plantsin which 
the parasitism has gone further, and has resulted in so 
great a modification of the plant body as to place them in 
another phylum. 
338. The Disk Lichens abound almost everywhere— 
on tree-trunks, rocks, old roofs, and in many regions upon 
the ground. ‘They are for the most part of a greenish- 
gray color, and hence are often called ‘‘Gray Mosses.” 
Other colors, as black, purple, yellow, and white, are also 
common. 
339. The plant-body of a Disk Lichen is composed of 
jointed, branching, colorless filaments, similar to those in 
assax>~=—SCst the ~ other fungi, but usually more or less 
DES compacted together into a thallus, or even 
ZA a branching stem. ‘They obtain their 
Fo%33.2,¢ nourishment from little green Myxophy- 
Fia. 95.—Section ceae or Chlorophycee to which the fila- 
of a Disk Lichen, 7% 
ments attach themselves parasitically. 
These little hosts, which at first live free in water or on 
moist surfaces, eventually come to live in the midst — 
of the moist tissues of the fungus parasite. They 
were formerly supposed to be parts of the lichen itself, 
and were called “‘gonidia,’”’ an objectionable term which 
is still in common use. 
340. Disk Lichens are all of rather small size, vary- 
ing from a millimeter or so, to 20 or 30 centimeters in 
length. For the greater part the plant-body is flattish, 
and adherent to the surface upon which it grows, but 
