Feb. 9, 1924 
Resistance of Wheat to Puccinia graminis tritici 
405 
seem reasonable. It is quite true that appressoria may be produced 
fairly abundantly on the surface of certain plants, without any subse¬ 
quent development of the germ tubes. The failure of the fungus to 
enter, even after appressoria have been formed, probably must be ex¬ 
plained on the basis of the absence of the necessary stimulus or possibly 
by the failure of stomata to remain open sufficiently long to enable 
the rust fungus to enter. 
There can be no question whatever but that there is a real physiologic 
or protoplasmic resistance. For instance, Khapli is resistant to all 
biologic forms of P. graminis tritici , though flecks practically always 
develop after inoculation. Naturally, therefore, the fungus must have 
entered the host tissue. It has been demonstrated beyond doubt that 
the cells of many resistant hosts are hypersensitive to the attacks of the 
fungus (2), (34), (5/). They are killed very quickly after the fungus 
comes in contact with them, and the hyphae of the invading fungus then 
die. 
It has been claimed that the physiology of the host plant can be 
changed rapidly and profoundly enough by altering environmental con¬ 
ditions to cause great variations in the protoplasmic resistance of the 
host to P. graminis . While this never has been demonstrated, certain 
observed facts seem to substantiate the claim. It frequently has been 
observed that the resistance of a variety apparently may change when 
conditions are altered. It also has been shown that young plants some¬ 
times are more susceptible than older ones. For instance, Stakman and 
Piemeisel (55) have observed that young plants of Agropyron smithii can 
be infected easily as a result of artificial inoculations. It is much more 
difficult, however, to infect older plants. It also has been observed that 
certain varieties of wheat, such as Kota and Acme, are susceptible to a 
considerable number of biologic forms in the greenhouse but that they 
seem to be fairly resistant in the field. This fact must be due either to 
physiologic or morphologic changes in the host. 
Extensive studies of the morphology of different varieties of wheat 
were made in order to ascertain whether there were any differences 
sufficiently great to account for the differences in rust resistance. It 
was found that the amount of sclerenehymatous tissue in the stem of 
Kota, Acme, and certain other varieties is very large in proportion to the 
amount of the chlorenchymatous collenehyma. As the rust can live 
practically only in the chlorenchyma, and as the only tissue in the stem 
which contains chlorophyll is the collenehyma, it is clear that the amount 
of collenehyma would determine to a Very considerable extent the amount 
of development which is possible for the rust fungus. In certain resistant 
varieties the sclerenehymatous tissues have developed to such an extent 
as to decrease considerably the amount of space in which the rust can 
grow. Sclerenehymatous strands extend from the vascular bundles to 
the epidermis arid a complete sclerenchyma sheath extends around the 
stem just outside the vascular buridles. The collenehyma bundles, 
therefore, are small and completely separated from each other. They 
are surrounded on three sides by thick-walled* lignified, sclerenehymatous 
fibers, through which the rust hyphae can not grow and from which they 
can not obtain nourishment. The size of the uredinia, therefore, is 
limited by the size of the collenehyma bundles. It is impossible for the 
rust to spread for any distance tangentially or radially. It can spread 
only longitudinally and this probably accounts for the fact that long, 
