Nov. is, 1920 Crownwart of Alfalfa Caused by Urophlychs alfalfas 315 



(6) figures of P. maculate (Melanotaenium alismatis) , the hypha con- 

 necting "corps central" and spore is even longer, exceeding here the 

 length of the "corps central"; and, as has been indicated above, an 

 entirely comparable figure is given by Ludi to illustrate conditions in 

 P. menyanthis. If these writers have not mistaken turbinate cells (or 

 their homologues) for resting spores and have not erred in relating the 

 latter to the wrong turbinate cells, it would appear that conspicuous 

 variability in length is characteristic of the connecting isthmus which in 

 Urophlyctis alfalfae and U. pluriannulatus is extremely short. 



Magnus emphasized the difference in anatomical effects produced by 

 species he referred to the genus Urophlyctis and by those he assigned to 

 Physoderma. The former cause hypertrophy and thickening of host 

 cell wall, while the latter leave the host tissue in an approximately normal 

 condition. Perhaps a distinction on such grounds would make the 

 classification of parasitic forms contingent in too large a measure on 

 reactions of the host plant to be admissible in a taxonomic sense. It 

 seems not improbable that further study of the plants now referred to 

 Urophlyctis, Physoderma, Cladochytrium, and perhaps a few other 

 related genera will reveal possibilities in generic regrouping based on the 

 more significant similarities and differences in morphology and develop- 

 ment. 



PATHOLOGICAL MORPHOLOGY 



It has already been stated that the fungus attacks primarily leaf scales 

 and leaves at a very early stage of development in the growing bud. 

 Only rarely has it been found to have penetrated to the axis in the 

 dividing undifferentiated tissue of the bud. The stimulative effect of 

 the fungus is limited strictly to the structure which has been invaded, 

 while other structures in the vicinity of the main axis and the axis itself 

 show retardation and often cessation of development. 



The first morphological change in the host consequent upon invasion 

 consists in a slight enlargement of the first cell entered so that it comes 

 to project both outwardly and inwardly against the underlying cells. 

 These underlying cells may also show a slight enlargement before they 

 are actually entered by the advancing fungus. The nuclei of the affected 

 cells enlarge notably, and the large deep-staining nucleoles persist for a 

 long time in the fungus cavities, their number serving as an index to the 

 number of host cells that have been destroyed. 



The fungus evidently gains access to new cells by the solution of thin 

 cell walls in advance of the growing turbinate cells. In early develop- 

 ment when a number of these fungus cells are advancing close together 

 in the same direction, the walls of the host cells are found dissolved 

 before the fungus comes in contact with them (PI. 55), thus precluding 

 the possibility of mechanical pressure as a factor in effecting the advance. 

 In later stages, however, when turbinate cells are fewer and more scattered, 



