XXIIL ] SUMMER RUST AND MILDEW. 149 



If we now cut across the larger of the two pustules 

 of summer rust, as illustrated at Fig. 73, and magnify a 

 thin transparent slice from the cut surface 200 diameters, 

 as was done with U. Eubigo-vera, D.C., we shall see it as 

 at Fig. 74. We now find that a typical pustule of 

 summer rust is so large that the page of our book is in- 

 sufficient for it. The engraving at Fig. 74 therefore shows 

 one-half the diameter of a sorus only, AB being the centre 

 line. The closely packed Uredo spores are supported on 

 somewhat longer pedicles than in U. Rubigo-vera, B.C., 

 and the spores themselves differ not only in colour but also 



A 



; -x 200^ 



FIG. 74. 



Transverse section through half a Pustule or Sorus of Uredo linearis, Pers. 

 Enlarged 200 diameters. 



somewhat in shape and size. They spring from a stratum 

 of involved jointed brownish mycelium. The constituent 

 cells of the wheat leaf are shown at Fig. 74, C, and 

 two organs of transpiration belonging to the leaf are shown 

 at DD. Two typical Uredo spores, supported on their 

 transparent pedicles, are enlarged 1000 diameters in Fig. 

 75. No individual spore in a sorus exactly agrees in 

 shape and size with another ; they vary, however, within 

 well-defined limits. Germination has commenced in the 

 smaller of the two spores at A. 



The Uredo spores fall very readily from their support- 



