XXIII.] 



SUMMER RUST AND MILDEW. 



151 



the yellowish granular contents of the Uredo spore pour 

 into the other and more strongly growing tube, BB, Fig. 

 76. In a day, or a day 

 and night, this stronger 

 thread will have grown in 

 a convolved fashion, as at 

 C, and will have attained 

 many times the length of 

 the spore from which it 

 started, and the whole of 

 the contents of the spore 

 will now be in the ger- 

 minal tube. When this 

 stage of growth is attained 

 a septum or stop grows 

 across the my celial thread, 

 as at D, and the dead and 

 empty spore case is cut 

 off from the living thread. 



If this process of ger- 

 mination is watched on 

 glass and not on a cuticle 

 stripped from a wheat leaf, it will be noticed that the 

 germ tube will flow into any little scratch or depression 

 on the glass, just as a brook gradually flows into the 

 lowest positions of a river valley, or as rain water makes 

 its way into our brooks by following little depressions in 

 the land surface. 



If, on the other hand, the process of germination is 

 watched on a shred of transparent epidermis torn from 

 a wheat leaf, the thread of mycelium will be observed 

 to follow the minute depressions formed where the con- 

 stituent cells of the epidermis meet, and by following these 

 fine depressions, as at E, the germ tube at length 

 naturally arrives at one of the minute mouths, organs 

 of transpiration, or stomata, belonging to the plant, as at 

 F. These are the lowest points on a leaf surface, and the 



B 



X-400JJ 



FIG. 76. Two spores of Uredo linearis, 

 Pers., germinating on a fragment of 

 the epidermis of a Wheat leaf. 

 Enlarged 400 diameters. 



