70 THE BUSTS OF GRAINS IN THE UNITED STATES. 



results on the rust attack might easily be erroneously attributed to 

 the action of certain chemical constituents of various fertilizers on 

 the rust itself. It seems probable that this is the case in 'the above- 

 cited results attributed to nitrogen-bearing fertilizers, viz, that the 

 fertilizer produced a very luxuriant growth on which the rust attack 

 would naturally be virulent. 



RESISTANT VARIETIES. 



CAUSES OF RESISTANCE. 



That some plants are far more resistant to the attacks of parasitic 

 fungi than others of the same genus or species has long been noticed, 

 and that this holds true with respect to grains is well established. 

 Some remarkably rust-resistant wheats, such as the durums and the 

 primitive einkorn wherever grown, Extra Squarehead in Sweden, 

 American Club in England, and Rerrarf and Ward's Prolific in Aus- 

 tralia, are well known. Some of these varieties, however, can not 

 be said to be universally rust resistant, as one variety may be resist- 

 ant to one or more species or biologic forms of rust in one country 

 but will not necessarily hold the same balance toward other forms of 

 rust, or in another country (51, p. 36; 39, pp. 340, 341; 44, p. 249; 

 43, pp. 141-144; 75, p. 27; 30, pp. 59, 60; 28, pp. 661, 662). Thus, 

 for instance, Squarehead is more resistant toward Puccinia glumarum 

 in Sweden than toward Puccinia triticina, and Rerrarf, while very 

 resistant in Australia, breaks down completely in North Dakota. 

 Numerous instances of this kind might be cited. 



It has not yet been established to what character of the plant this 

 elusive and seemingly erratic resistance is due. From a large num- 

 ber of inoculation experiments with the brown rust of bromes and 

 from detailed histological investigations of the hosts, Ward (98, p. 

 303) found that there is absolutely no relation between differences 

 in the morphology of the brome varieties expressed in length of hairs, 

 number and size of stomata, thickness of epidermis, etc., and rust 

 resistance. He concluded: 



Resistance to infection of the immune or partially immune species and varieties 

 is not to be referred to observable anatomical or structural peculiarities, but to inter- 

 nal, i. e., intraprotoplasmic properties beyond the reach of the microscope and simi- 

 lar in their nature to those which bring about the essential differences between species 

 and varieties themselves. 



In the study of resistant and nonresistant wheats the same author 

 (102, pp. 38, 39) showed that rust spores germinate on both suscep- 

 tible and resistant varieties and gain entrance to them through 

 stomata, but in the resistant varieties further progress is checked by 

 the rapid deterioration and collapse of host cells around the entering 

 fungus, while in the nonresistant varieties the host cells remain turgid 



216 



