Feb. 9,1924 Resistance of Wheat to Puccinia graminis tritici 
407 
resistance but only a change in the opportunities of the rust to develop, 
due to a spatial limitation of the growth of the organism. 
SUMMARY 
1. In addition to fundamental protoplasmic resistance, wheat varie¬ 
ties may possess other means of defense against Puccinia graminis. 
2. The number of leaf hairs and the size and number of the stomata 
can not be considered important in influencing the entrance of the germ 
tubes. When there is only a small amount of inoculum, the large num¬ 
ber of hairs on some varieties may prevent some of the germ tubes from 
reaching the stomata and growing through the stomatal slits. 
3. Stomatal movements may have some influence on the entrance of 
germ tubes. The stomatal movements of different varieties of wheat 
apparently are affected differently by environmental conditions. 
4. The mycelium of P. graminis within the host is limited almost 
entirely to chlorenchymatous tissue. As the only important chloren- 
chymatous tissue of the stem is the collenchyma, the rust mycelium can 
grow only in this tissue. In some varieties of wheat there is such a 
large amount of sclerenchyma that the band of collenchymatous tissue 
is broken up into small bundles. The extent of mycelial development, 
therefore, is limited to these relatively small areas. 
5. The amount of sclerenchyma is not the same in the stems of different 
varieties. In some there is a very large amount and in others relatively 
little. The amount of collenchyma is approximately inversely propor¬ 
tional to the amount of sclerenchyma. In those varieties, therefore, in 
which there is a large amount of collenchyma, large uredinia are likely 
to be produced, while in those varieties in which the collenchyma bun¬ 
dles are small, the uredinia are likely to be narrowly linear. Varieties 
in which there is a great deal of sclerenchyma are likely to be injured 
less by rust, as there is a mechanical limitation to the spread of the 
mycelium. 
6. The relative proportion of sclerenchyma to collenchyma in a given 
variety may be altered by the use of fertilizers. 
7. Excessive fertilization with nitrogen has a tendency to decrease 
the amount of sclerenchyma in proportion to the amount of collenchyma. 
For this reason, plants heavily fertilized with nitrogen may be more 
severely injured by rust than those which have not been so fertilized. 
8. The fact that the seedlings of some varieties appear to be more 
susceptible to certain biologic forms of P . graminis than are the older 
plants can be explained by differences in morphology between the seed¬ 
lings and the mature plants. There is a greater amount of scleren¬ 
chyma in the mature plants than in the seedlings and this constitutes a 
mechanical restriction on the growth of the mycelium. 
9. There are differences in the physicochemical properties of the sap 
of different wheat varieties. It has been impossible, however, to make 
a definite correlation between these properties and rust resistance. 
10. The differences in the reaction of wheat varieties to different bi¬ 
ologic forms of P . graminis tritici appear to be due entirely to physi¬ 
ologic causes. 
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