184 THE ROYAL SOCIETY OF CANADA 



Distribution and Frequency. 



The geographical distribution of the various biologic forms is still 

 imperfectly known. However, tentative maps have been prepared 

 showing the areas in which the more frequently occurring forms have 

 been collected. These have been made by the simple expedient of 

 connecting with a broken line the more outlying points at which 

 collections have been made. The boundaries so arrived at will, no 

 doubt, be extended by further exploration. Indeed, it seems probable 

 that no southern boundary exists, and that in some cases, at least, 

 the northern boundary may coincide with the limits of the wheat- 

 growing area. On the other hand, it is probable that the frequency 

 of occurrence will diminish towards the outlying parts of the areas 

 involved, although, in special cases, natural barriers may possibly 

 interpose an abrupt limit. It must be noted, further, that the date 

 of occurrence in a given locality possibly varies with remoteness from 

 the point of origin of the infection. This may explain why collections 

 in the Calgary to Edmonton district of Alberta are rarely possible 

 before September. Such conditions are, of course, extremely for- 

 tunate for the region concerned, since infection occurring thus late 

 cannot do any serious damage to the crop. 



In Fig. 1, A. is shown the area from which collections of a rather 

 virulent form, XVII, have been made. This is by far the most widely 

 distributed of all the forms isolated. It is found in 26 distantly 

 separated districts of Manitoba, Saskatchewan, and Alberta, embrac- 

 ing a variety of climatic conditions, especially in regard to rainfall. 

 As already pointed out, the area delimited on the map may safely be 

 taken as a very conservative index of distribution; in many places 

 outside this area, but one collection of rusted material has been made. 

 The persistance of Form XVII is shown by the fact that once it has 

 been found in any locality, collections in one or more succeeding 

 seasons have rarely failed to demonstrate its recurrence. 



Within the same area are included, of course, many other forms ; 

 eight have already been found. 



In Fig. 1, B. is shown the area in which have been found 6 forms 

 (I, IX, XVII, XXI, XXIX, XXX) all of which cause the same 

 infection upon all the bread wheats, as may be seen by reference to 

 Table III. That is not to say that these 6 forms are identical, since, 

 as may also be seen, they vary greatly in their parasitism towards 

 other varieties. The great importance of this group will be apparent 

 when it is pointed out that they include 70 per cent, of the rust 

 collections made, and cover practically the whole area now occupied 



