THE FUNGI WHICH CAUSE PLANT DISEASE 311 



only three are of economic importance. By Engler and Prantl, the 



genus is not separated from Ustilago. 



Sphacelotheca sorghi (Lk.) Cl.^'- ^^ ^^s, m, 144 



Sori usually in the ovaries or stamens forming oblong to ovate 



bodies 3-12 mm. in length (rarely fusing the very young spikelets 



into irregular forms), protected for 



some time by a false membrane 



upon the rupture of which the 



olive-brown spore-mass becomes 



scattered, leaving naked the dis- 

 tinct columella of plant tissue. 



The sterile cells of the membrane 



break up to some extent into 



groups, hyaline, oblong to sub- 

 spherical, chiefly 7-18 /x in length; 



spores subspherical to spherical, 



smooth, contents often granular, 



5.5 — 8.5 fi in diameter. 



On Johnson grass and sorghum 

 throughout the world. The young 

 pistil and usually the stamens as 

 well are displaced by the fungous 

 mycelium, the two being often 

 blended together. The spores ger- 

 minate readily in water, either 

 when fresh or a year old, showing 

 papillae in from three to ten hours. 

 The promycelium is 2 to 3-septate 

 and from the ends of one or more 

 of its cells narrow tubes appear. 

 These later fuse with the adjacent cell, forming the «' buckle 

 joints." Either infection tubes or sporidia may also arise from 

 the promycelium. Infection is possible only with young plants. 

 The mycelium in the host plant grows rapidly into long irregu- 

 lar, hyaline, thin-walled threads 2-4 mm. thick, which run through 

 and between the cells. It is most abundant in the parenchyma, 

 advancing especially through the pith region with the growth of 

 the host. The young ovaries and stamens are eventually reached 



Fig. 225. — S. sorghi, cross-section 

 through base of young infected 

 body (ovary), a, false membrane 

 of epidermal cells and sterile'my- 

 celium, 6, mature spores, b ', im- 

 mature spores, c, columella. After 

 Clinton. 



