in the Brontes and their Brown Rust. 303 
should do so. I have accordingly gone into this question 
of the structural peculiarities of the host-plant in a manner 
more searching and thorough than hitherto, and have, I believe 
for the first time, devised methods of attacking the question 
which promise accurate results, which results, moreover, are 
perhaps somewhat surprising in their nature. 
This part of the investigation has, as will readily be under- 
stood from the sequel, proved a very laborious, not to say 
tedious, business, but it was obvious that it had to be done ; 
and although I am not prepared to state that all the numerous 
measurements and calculations are accurate and final, it will 
be seen that they hang together sufficiently firmly to make 
a chain of evidence which seems to support the main con- 
clusion, which is, that the resistance to infection of the 
‘ immune ’ or ‘ partially immune ’ species and varieties is not 
to be referred to observable anatomical or structural peculi- 
arities, but to internal, i. e. intra-protoplasmic, properties 
beyond the reach of the microscope, and similar in their 
nature to those which bring about the essential differences 
between species and varieties themselves. 
The problem which I set myself was, to investigate as 
thoroughly as possible the sizes, numbers, and distribution 
of the stomata, hairs, chlorophyll-tissue, vascular bundles, 
sclerenchyma and other structural units of the leaf, and to 
see if in any two or more species or varieties of Brome such 
structural relations showed differences at all compatible with 
differences in their predisposition to the disease or to their 
immunity from infection. 
In order that no avoidable complications should be intro- 
duced owing to variations on the part of individuals grown 
under different conditions, or in different leaves or parts of 
leaves, I adopted the precaution of selecting for examination 
of these structural factors the same part of the same leaf, 
grown under the same conditions. Not only so, I took this 
leaf from the pots of two of the most conclusive sets 
of experiments, viz. Nos. 774 and 775 (see Table XXII), 
in the following manner. 
