150 BOTANY 



Mycorhiza. It has been found, also, that the roots of many of the 

 higher plants are infested by a Fungus, whose delicate filaments 

 apparently serve the purpose of root-hairs ; the Fungus apparently 

 receives for its services a certain amount of nourishment from the 

 roots with which it is associated. These Eoot-fungi have been named 

 Mycorhiza, but as they are always sterile, it is not known with what 

 other Fungi they are related. 



Parasitic Fungi, as a rule, are very injurious to the host, and are 

 the principal causes of plant diseases. Their growth within the tis- 

 sues of the host not infrequently causes an abnormal growth of its 

 cells, causing gall-like swellings, as in the case of the Cedar-rust 

 already referred to. It is not unlikely that these growths are due to 

 the action of certain secretions (enzymes) similar to those by means 

 of which the Fungus is enabled to dissolve and penetrate the cell- 

 walls of the tissues in which it is growing. It is thus that some ger- 

 minating spores make their way through the outer cells of the host 

 and reach the inner tissues. 



Fermentation. Fungi which live upon dead matter, by its decom- 

 position play a similar though less important role to that of the 

 Bacteria, in reducing the organic compounds to simpler ones. One 

 characteristic form of organic decomposition is the alcoholic fermen- 

 tation of sugary solutions through the agency of certain low Fungi, 

 especially the Yeast-fungi. 



Structure of Fungi 



A small number of Fungi are unicellular, but much the larger num- 

 ber are composed of filaments, or " Hyphae," which are massed into 

 the vegetative body, or " Mycelium," upon which are borne the vari- 

 ous reproductive bodies. The mycelium may be a delicate weblike 

 structure, or the hyphse may be densely matted together so that the 

 mycelium has an almost leathery texture. 



In the Phycomycetes, the hyphse are nearly or quite undivided ; but 

 the hyphse of the higher Fungi are divided by transverse septa, which 

 are generally formed in regular succession from an apical cell. 



Cell-wall. The membrane of the hyphse may be composed of ordi- 

 nary cellulose ; but as it becomes older, there is usually a change into 

 fungus-cellulose, which differs slightly from that of the green plants. 

 In some cases, too, it becomes very hard, and the Fungus may be 

 almost woody in texture, as is seen in the so-called sclerotium of the 

 Ergot of Rye, for instance. 



Protoplast. The protoplasm may fill up the cells completely, but 

 usually there are large vacuoles. No chromatophores are present, 

 and the nuclei are small, and often difficult to demonstrate, but not 

 essentially different from those of other plants. In the very long 



