(4i9) 



but there are a number of forms where the telia occur on the leaves 

 or young branches in which there is some reason for believing that 

 the maturation is annual, spores developing in the spring from an 

 infection of the previous summer. It is altogether probable that 

 the development of the fungus may not require any definite length 

 of time with respect to the seasons directly, except in so far as it 

 is dependent upon the growth of the host, which is, of course, in- 

 fluenced by the seasonal changes. It is further possible that the 

 time and place of infection may have an influence upon the rate 

 of growth of the mycelium. This is an interesting field in which 

 further experimental data are desirable. 



4. Pathologic and Economic Importance 



Pathologic Effects 

 The anatomic changes induced in the host-tissues by the my- 

 celial growth of the species of Gymnosporangium are in general 

 more noticeable in the telial than in the aecial hosts. Some of 

 the species, it is true, produce their sori on the leaves of the cedars 

 and junipers without causing material deformations, but the ma- 

 jority of the species cause twig-deformations either of the nature 

 of gradual swellings or of abrupt gall-like swellings. Other species 

 induce malformations of the branch-systems commonly known as 

 witches' brooms. Woernle (1894) has especially investigated 

 the changes induced in the internal morphology of the telial hosts. 

 In the case of G. clavariaeforme, which causes gradual fusiform 

 swellings of the branches, he found increased growth in the xylem, 

 phloem, and cortex, the largest increase taking place in the phloem. 

 The medullary rays were found to be more numerous and somewhat 

 higher than usual; the wood parenchyma was more abundant; 

 the tracheides no longer followed a straight course and had nu- 

 merous intercellular spaces between them. The mycelium filled 

 the phloem and cortical regions and sometimes formed masses in 

 the intercellular spaces. An examination by the same investigator 

 of similar swellings induced by other species of Gymnosporangium 

 revealed the same general pathologic effects in the tissues. In 

 most of the hypertrophied areas the limits of the annual growth- 

 rings were difficult to make out, and in the swellings produced by 

 G. Sabinae a yellow pigment was found deposited in the walls of 

 all the elements. It is interesting to note that the scar left by the 



