6 (JSTILAGO AVENAE AND USTILAGO LEVIS 



turn upward and grow between the cells, obtaining- their food by means 

 of hailstorm. They traverse the primary node through an opening be- 

 tween the vascular bundle tips and the insertion of the first leaf. Here 

 they branch and are thus established in the growing apex of the suscept. 

 After the tissues of the primary node have become mature, the patho- 

 gen;' is not able to pass through the node and reach the meristem above. 

 The mycelium originating from the hyphae that have successfully 

 passed through the primary node, keeps pace with the growing point as 

 this is pushed upward. The fungus causes a mild stimulation of the 

 suscept cells, which accounts for the slightly more vigorous growth of 

 the infected plants often observed during the period of optimum vege- 

 tation. 



According to Butler (1918:179-180), the growing point may some- 

 times grow away from the pathogene. and the mycelium thus left behind 

 becomes disorganized, breaks up into segments, and disappears in the 

 older tissues, just as it does even in the culms in which the pathogene 

 succeeds in maintaining itself in the growing point. 



As the culms elongate, the pathogene accompanying the growing 

 point at last reaches the developing flowers. Here the mycelium 

 branches profusely, becomes swollen and knobbed, and absorbs the dif- 

 ferentiating suscept tissues. The chlamydospores are formed inside the 

 terminal branches of the mycelium, the walls of which at length gelati- 

 nize and disappear. Thus, in place of the normal flower with its glumes, 

 there is produced a mass of black or dark brown spores ready for dis- 

 semination. The degree to which normal tissues are replaced varies, 

 sometimes both the flowers and the glumes being completely replaced, 

 sometimes the glumes remaining partly or entirely intact. It is com- 

 monly observed also that only a part of the panicle is smutted, in which 

 case it is always the lower part that is affected. 



LIFE HISTORY OP USTILAGO LEVIS 



The life history of Ustilago levis is held to be very similar to that of 

 r. avenae. Much of this similarity has been assumed. The difference 

 in appearance of plants smutted by U. levis and by U. avenae was 

 brought out by Kellerman and Swingle (1890) when they distinguished 

 between the two pathogenes. With U. levis, the heads of the oat plant 

 are not so completely destroyed. The glumes remain intact and it is 

 often difficult to know whether the spores are present without breaking 

 open the glumes. Clinton (1900:297) says, "These enveloping parts 

 furnish such protection against dissemination of the spores that fre- 

 quently the smutted flowers may be found in badly smutted grain after 

 it is threshed. ' ' He states also that experiments with covered smut show 

 that it is the spores which in the field have succeeded in falling between 

 the open glumes at flowering time, that are blamable for infection. 



