PART 11: 
resistence it is not until haustoria have penetrated into the cells of 
the wheat plant that the cells die. In such cases a little food ie 
extracted and the rust plant is able to mature and form a very small 
pustule surrounded by a yellowish white area. Such a variety of 
wheat would be highly resistant but not immune to the biologic form 
of rust which attacked it. Degrees of resistance to any biologic form 
depend on the relative size of the pustule to the yellowish white area 
around it. 
In some cases it is possible that the germ tube has such an effect 
on the guard cells that they close as soon as the germ tube comes 
into contact with them. In such a case no flecks are formed as the 
germ tube has not been able to enter. 
No satisfactory explanation has been given of this form of 
resistance to rust. In this connection the following quotation from 
a paper by C. B. Hurst is of interest: ‘‘ It seems quite obvious that 
this must be due to physico-chemical relations between the host and 
the pathogene. As the pathogene cannot be grown in artificial 
culture media, a study of the problem is somewhat difficult. . The 
most promising method of attack would seem to be to ascertain 
whether there are consistent differences in the physicochemical 
properties of different varieties. Determinations therefore were made 
of hydrogen-ion concentration, sugar content, etc. None of the 
observed differences seemed to be consistent or great enough to 
account for the differences in resistance. While it may be significant 
that the sugar concentrations in different varieties varied somewhat, 
too much importance should not be attached to these results. A 
study also was made of the reaction of spores of different biologic 
forms to certain physicochemical factors. It was found that the 
spore germination of two biologic forms was affected differently by 
hydrogen-ion concentration and by temperature. The form with the 
widest host range was able to withstand a wider range of variation 
than was the form with the narrower host range. This fact probably 
is of some significance, but just how much it is impossible to say.’’ 
1t has often been thought that some varieties of wheat are 
morphologically resistant to rust. It has been shown that the number 
of leaf hairs, the number of stomata and the size of the stomata 
openings have no practical effect either way on the entry of rust germ 
tubes into the leaf. Stomatal movements do have an effect. That 
is to say appresoria may be formed over the stomata but owing to 
conditions of plant nutrition the stomata do not open to admit the 
germ tube. An unbalanced excess of nitrogen and a lack of silicic 
acid in the soil tend to make wheat more susceptible to rust. The 
growth of the wheat is luscious, moisture is retained permitting the 
easier germination of the rust spore, transpiration is greater causing 
the stomata to open and allow the entrance of germ tubes, the cel 
walls are thinner allowing the haustoria to penetrate more easily, and 
the proportion of chlorenchymatous to sclerenchymatous tissue is 
increased, thus allowing it more room for the development of uredinia. 
The morphological conditions above enumerated depend chiefly on 
environmental conditions and not on varietal characteristics and too 
much attention should not be paid to wheats resistant from such 
causes 
- Rust can ‘only develop in chlorenchymatous tissue. = Therefore 
varieties in which collenchyma areas are small and separated by large 
102