452 
Journal of Agricultural Research 
Vol. XXIII, No. 6 
investigators have not been able to duplicate his results. Biffen (j) is 
positive that such “ bridging or increase in virulence did not occur among 
any of his many hybrids in England. Stakman and his associates (45-53) 
in America, after long and careful work with stemrust on its many hosts, 
have failed to find any indication of “bridging” or increased virulence of 
the parasite. They grew it on hybrids of resistant X susceptible wheat 
varieties from the F t , F 2 , and F 9 generations. They used different 
physiological races of the parasite and many species, including several 
genera of host material. In no case did they find an intermediary host 
that would increase the virulency of the rust. On the other hand, they 
found that the parasite became sickly when cultivated on a resistant 
host and often could not be maintained more than a few generations. 
According to Vavilov (38) “bridging species” are found in Puccinia 
Symphyti Bromorum F. Mull. (M. Ward and Freeman), Erysiphe graminis 
DC. living on Bromus (E. Salmon), in P. graminis forma sp. tritici (Free¬ 
man and Johnson), and Sphaerotheca Humuli on Alchemilla (Steiner). 
It will be noted that these few cases deal with relatively weakley special¬ 
ized fungi, when compared with the smuts, among which biologic races 
of different infecting power are known to exist; and the hosts used 
contain races varying from complete susceptibility to immunity. The 
conflicting accounts of American investigators may be explained on 
similar grounds. For example, Johnson (20) was able to transfer 
timothy rust Puccinia phleipratensis Erikss. and Henn. to barley by 
means of oats as a bridging host when it could not be transferred from 
timothy direct, but Stakman and Jensen (47) transferred timothy rust 
successfully to oats, barley, rye, wild oats, oat grass, orchard grass, 
wild rye, rye grass, and rough brome, without the use of a bridging host. 
The biologic races of hosts and parasites, differing in resistance and 
virulency and the difficulty of keeping pedigreed cultures of each not 
only make accurate work difficult but make it almost impossible for two 
investigators working in different parts of the country to corroborate 
each other’s work. “Wheat” and “stemrust” are terms not sufficiently 
specific for work of this kind. The particular variety of wheat and the 
biologic race of the rust must be known. 
Plants vary also in their resistance to the attacks of insects. Tors- 
sell (56), in Sweden, found certain winter wheats that were not harmed 
by the six-spotted leafhopper while other varieties in the same field 
were destroyed. In 1916 in the variety test of field peas planted at the 
Washington Agricultural Experiment Station, the seed of Alaska peas 
was infested with pea weevil. At harvest time the Alaskas contained 
more infested than good seed, while the Bangalias growing beside them 
contained only a trace. The adult weevil is winged but evidently 
preferred the Alaskas. It is a common observation among potato 
growers that the Colorado potato beetle will destroy certain varieties 
before attacking others. For example, if Early Rose and Rural New 
Yorkers are planted in alternate rows in a garden, the beetles will strip 
the leaves of the Early Rose before harming to any appreciable extent 
the Rurals. But perhaps the most striking example of resistance to 
insect attack is that of certain grapes resistant to the plant louse 
Phylloxera vitifoliae Fitch, described by Bioletti (4). When resistant 
and susceptible varieties are crossed, resistance seems to be dominant. 
Such hybrids have become the foundation stock for grafting with the 
choice wine, table, and raisin grapes of Europe. Phylloxera feeds on the 
