26 



THE RUSTS OF GRAINS IN THE UNITED STATES. 



kept continuously for check purposes. All precautions possible to 

 ordinary greenhouse work were taken to avoid accidental infection 

 and the mixing of cultures. The results given in the table below 

 show the measurements of (1) the original material — that is, of barley 

 rust on barley and wheat rust on wheat; (2) the same rusts after 6 

 inoculations had been made on new hosts; and (3) after 17 inoculations, 

 covering a period of almost one year. The spore measurements were 

 carefully made from typical spores. Where 50 spores were measured, 

 they were taken from at least 5 different pustules and from 2 or 3 

 leaves. Wliere 10 were used they were selected from normal, mature 

 pustules. The difference in size between spores from mature and 

 immature pustules is quite marked, tlie immature being smaller than 

 the mature. As the variation in width of the spores from different 

 hosts is ver}^ slight, measurements of 10 may show a small difference 

 from the measurement of 10 original spores toward either plus or 

 minus. 



Table II. 



-Change in morphology of the uredospores by (ntlfivnfion of stem, nist from 

 wheat on barley and from barley on wheat. 



This table shows that in the original material the wheat-rust 

 uredospores are 0.69 /i wider and 2.82 jj. longer than the corresponding 

 barley-rust spores. As stated above, the difference in width is too 

 small (a little more than 0.5 /i) to allow of safe conclusions as to its 

 variations. After 6 successive infections of each rust on the other 

 host, the wheat rust had lost an average of 1.2 /i in length, tlie width 

 remaining practically the same ( + 0.02 ^(), Tlie barley rust, on the 

 otlier hand, had gained in length 1.71 ^, tlie width running ])racti- 

 cally the same ( + 0.07 n). The two rusts at this time gave almost 

 identical measurements, viz, wheat rust on barley IS. 17 /< by 30.13 fi 

 and barley rust on wheat 17.53 fi by 30.22 ;<. After 17 successive 

 intervening inoculations (almost a year from the time of the collection 



216 



