262 ESSENTIALS OF BOTANY 
and some of them parasitic. A few may live in either 
fashion at will. The mycelium through which the plant 
obtains its nourishment is often so fine and inconspicuous 
that it escapes notice, and the fruiting portion is com- 
monly. thought to spring directly from the earth, bark, 
wood, or other substratum to which it is attached. The 
reproduction is wholly asexual. 
Gill fungi are of consider- 
able economic importance. A 
good many are edible (though 
others which resemble them are 
actively poisonous). Such par- 
asitic species as Armillaria mel- 
lea (Fig. 185) are very injurious 
to timber. This fungus sends 
its mycelium through the bark 
or between the bark and wood 
of trees and often causes the 
death of the host. f 
A large shelf-fungus (TZ'ra- 
Fie. 187. Part of the Preceding mctes Pin) of a closely related 
Figure. (x about 300.) group (Fig.184) is very destruc- 
C, layer of cells immediately under tive to trees of the Pine family : 
the hymenium. s,s’, s”, three The lack of sexual reproduc- 
wee Beet ST tren ae organisms as compli- 
cated as the gill fungi seems 
to be evidence that these forms are degenerating. The 
same conclusion is suggested by the occurrence of what 
appears to be imperfect reproductive apparatus of Puccinia 
on the upper surfaces of infected barberry leaves. There 
is much other evidence of the same sort, and it all agrees 
with the supposition that fungi are degenerate descendants 
