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 algse 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 thnUophytic 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 alga? (commonly colonies of Pleuro- 

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

 among Pleurococcus cells germinates, and the hyphse attach- 

 ing themselves to the alga3 absorl) food materials from them 

 but not generally to an injurious degree. This is shown by 

 the fact that the alga? seem to thrive quite as well as before, 

 dividing repeatedly, while the hypha grow luxuriantly into a 

 mycelium which soon envelopes the algse 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 alga! colonies (r/, g) 

 mingled with the hypha3, and, covering all, a firm rind (r, r) 



