THE FUNGI 



89 



that in order for the plant to complete its life-cycle, it 

 was necessary that it should grow in turn upon both 

 hosts. 



In the eastern United States there is another very 

 conspicuous case of this heteroecism in the fungus caus- 

 ing the enlargements 

 on the twigs of the 

 red cedar known as 

 " cedar-apples " (Fig. 

 23, A). These mor- 

 bid growths are due 

 to the attacks of a 

 fungus (Gymnospo- 

 rangium) related to 

 the wheat-rust, and 

 in the spring the 

 large orange-colored 

 masses of spores are 

 exceedingly conspic- 

 uous, especially after 

 a rain, when the 

 gelatinous mass in 

 which they are envel- 

 oped swells up. The 

 spores in these masses 

 (B, C) give rise on 

 germination to sec- 

 ondary spores which 

 germinate at once in 

 case they fall upon 

 the young leaves of the wild crab-apple or hawthorn, 

 but will not grow upon the cedar. The fungus pro- 



FIG. 23 ( JScidiomycetes) . A, a branch 

 of red cedar attacked by a parasitic 

 fungus (Gymnosporangium), forming 

 the excrescence known as a "cedar- 

 apple"; sp, masses of spores growing 

 out from the surface of the cedar-ap- 

 ple; B, two spores of Gymnosporan- 

 gium, one of which is beginning to 

 germinate ; pr, the young promycelium ; 

 C, a germinating spore which has given 

 rise to a promycelium from each cell; 

 the secondary spores, x, produced upon 

 the promycelium do not germinate upon 

 the cedar, but produce upon the haw- 

 thorn the so-called "secidia," or clus- 

 ter-cups; D, a leaf of cockspur thorn, 

 with two groups of cluster-cups, ae ; 

 E, section through an fecidium of 

 another rust (Uromyces caladii). 



