504 LIFE-HISTORIES 



the vegetative system of the al,e;al ancestor ; aw 1 as the degeneration 

 proceeds, the reproductive system, whereby kiusliip 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 algie and fungi, making allowance for the probalsle ex- 

 tinction of man)' forms, and for a considerable evolution of fungi 

 as fungi. For further accounts of these evolutionary interpreta- 

 tions the student must he referred to more special works. 



The name fungus has been variously restricted l)y different 

 writers. As here used it is taken in the widest sense as in- 

 cluding all thdUophytic kysierophijtea, of which about 40,000 

 species have been described. 



Fungi are of great economic importance, manj' of the 

 saprophytic forms being, as we have seen, highlj' beneficial 

 as agents of dcca}'; while, on the other hand, parasitic forms 

 are often exceedinglj' harmful. Nearly- all the diseases of 

 cultivated plants which so seriousl.y affect the pursuit of 

 agriculture are due to fungal parasites. A scientific stud.y of 

 these, however, has led to the discovery of means of defense 

 ■which iuive 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 tlie 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 alga; (commonly colonies of Plcuro- 

 coccus), imprisoned by its mycelium. A lichen spore falling 

 among Pleurococcus cells germinates, and the hyphae attach- 

 ing themselves to the algai absorb food materials from them 

 but not generally to an injurious ck'grec. This is shown by 

 the fact that the algae seem to thrive ciuite as well as before, 

 dividing rei)eatedly, while the hyplue grow luxuriantly int(i a 

 mj-cclium wiiich soon envelopes the algce completely. A well- 

 developed lichen such as "Tc(>land moss," for example, shows 

 a compound thallus, in which a marked differentiation of 

 parts may be observed (Figs. IGl, 332-335). At the middle 

 is a layer of loose cottony mycelium (Fig. 335, in) on the 

 bordc'rs of which are irregular layers of algal colonies {g, g) 

 mingled with the hyphffi, and, covering all, a firm rind (r, r) 



