144 



SYSTEMATIC BOTANY. 



Pig. 260), or many-celled (as in Phragmidium). They 

 rupture the epidermis and become exposed, but generally 

 remain attached to their host-plant during the winter. In 

 the spring, they germinate by sending out from each cell 

 a jointed filament, called the promycelium. In small 

 branches of the promycelium, small terminal cells, or 



sporidia, are formed. 

 These are carried 

 about by the wind, 

 and germinate on 

 the proper host- 

 plant. They send 

 their filaments into 

 the parenchyma of 

 the leaf, from which 

 a mycelium proceeds 

 that gives rise to an 

 secidium, and so on, 

 as before described. 

 In some species all 

 the stages may grow 

 on the same plant; 

 more often the seci- 

 ^™ dial stage is found 



on one plant, and the other stages on some other one ; or, 

 in yet other species, each stage may have a different host- 

 plant. The Wheat Rust belongs to the second group; 

 its seeidial stage occurs only on the Barberry leaves, and 

 the uredo-spores and teleutospores are found on Wheat 

 and other Graminem. 



Fig. 260. Cedar-Apple, Gymnosporangium (or Podisoma") macroptis, on yuni- 

 perw Virginiana; tel^ teleutospores, highly magnified. 



