504 LIFE-HISTORIES 
the vegetative system of the algal ancestor; and as the degeneration 
proceeds, the reproductive system, whereby kinship is most plainly 
revealed, loses even the last vestige of sexual organs. Many interest- 
ing attempts have been made, however, to correlate the various 
classes of algee and fungi, making allowance for the probable ex- 
tinction of many forms, and for a considerable evolution of fungi 
as fungi. For further accounts of these evolutionary interpreta- 
tions the student must be referred to more special works. 
The name fungus has been variously restricted by different 
writers. As here used it is taken in the widest sense as in- 
cluding all thallophytic hysterophytes, of which about 40,000 
species have been described. 
Fungi are of great economic importance, many of the 
saprophytic forms being, as we have seen, highly beneficial 
as agents of decay; while, on the other hand, parasitic forms 
are often exceedingly harmful. Nearly all the diseases of 
cultivated plants which so seriously affect the pursuit of 
agriculture are due to fungal parasites. A scientific study of 
these, however, has led to the discovery of means of defense 
which have enabled farmers to increase their crops enor- 
mously in recent years. 
185. The spore-sac lichens (Class Ascolichenes). After 
long study and careful experimenting in the culture of lichens 
botanists have reached the strange conclusion that what 
were at first regarded as individual plants are in reality 
communities each consisting of a fungus (mostly spore-sac 
fungi), parasitic upon algze (commonly colonies of Pleuro- 
coccus), imprisoned by its mycelium. A lichen spore falling 
among Pleurococcus cells germinates, and the hyphe attach- 
ing themselves to the alge absorb food materials from them 
but not generally to an injurious degree. This is shown by 
the fact that the algee seem to thrive quite as well as before, 
dividing repeatedly, while the hyphz grow luxuriantly into a 
mycelium which soon envelopes the alge completely. A well-. 
developed lichen such as ‘‘ Iceland moss,”’ for example, shows 
a compound thallus, in which a marked differentiation of 
parts may be observed (Figs. 161, 332-335). At the middle 
is a layer of loose cottony mycelium (Fig. 335, m) on the 
borders of which are irregular layers of algal colonies (g, g) 
mingled with the hyphe, and, covering all, a firm rind (r, 7) 
