270 



THE FUNGI 



and are believed to be of great assistance to them in their work 

 in the following way. It is necessary for the roots, of course, to 

 establish a close relation to the moisture of the soil in order to 

 obtain water for the green parts of the plant above ground. The 

 surface of the older portions is without root hairs and is sur- 

 rounded by a hard outer layer which cannot come into very 

 close contact with the minute moist soil particles. But it is 

 thought that the fungal filaments act as root hairs, and perhaps 

 through them the root can absorb a much greater quantity of 



water and can well afford to give 

 them what nourishment they require 

 in exchange for such valuable 

 services. It is probable that most 

 trees and many others of the larger 

 plants have formed this partnership 

 with the fungi. The kinds of fungi 

 concerned with mycorrhizas are not 

 well understood, but some of them 

 are known to be the mycelia of 

 toadstools and puffballs. The sac 

 fungi also furnish notable examples 

 in the truffles (Sec. 269). The my- 

 corrhiza relationship is an excellent 

 illustration of symbiosis (which 

 means a living together), for two 



FIG. 242. Mycorrhiza surround- 

 ing the tip of a beech root 



After Pfeffer 



organisms exist here in intimate physiological association and 

 both apparently receive benefit from the partnership. 



279. Summary of the basidia fungi. The relationships 

 between the different groups of the Basidiomycetes cannot be 

 discussed further than to state that the promycelium of smuts 

 and rusts, with its sporidia, is believed to correspond to the 

 basidium with its four spores. There are two small groups called 

 the jelly fungi (orders Auricularales and Tremellales), includ- 

 ing the rather common Jew's-ear fungus, whose basidia become 

 divided into four parts. In the Jew's-ear fungus the basidium 



