Feb. 9,1924 
Resistance of Wheat to Puccinia graminis tritici 
39i 
passed through acid alcohol, cleared in carbol-turpentine, passed through 
xylol and then mounted in balsam. Light staining with iodin clearly 
brought out the chlorophyll-bearing areas of the stem. 
The wheat stem consists of a hollow cylinder, except in solid-stemmed 
varieties. (PI. 2, A and B.) The epidermis, of course, consists of a 
single layer of cells, the inner walls of which are sometimes lignified. 
The number of stomata per unit area of surface differs in different 
varieties. Just beneath the epidermis is the chlorenchymatous collen- 
chyma. This sometimes extends in an almost continuous band around 
the entire stem, although it usually is interrupted by strands of scleren- 
chyma. The collenchyma cells then are aggregated into isolated bundles, 
the size and number of which vary considerably in different varieties. 
The vascular bundles are arranged in a more or less definite ring in the 
thin-walled parenchymatous tissue of the fundamental ground tissue. 
Toward the inside is the pith. The bundles may be arranged in one or 
more rings; usually there are about 15 large bundles and a variable 
number of smaller bundles outside of the larger ones. A more or less 
continuous sheath of sclerenchymatous fibers extends between the differ¬ 
ent bundles and outside of them. Strands from this sheath often extend 
from the bundles through the collenchyma to the epidermis. It is par¬ 
ticularly important to remember that the only chlorenchymatous tissue 
in the stem is the collenchyma. These extensions of the sclerenchyma¬ 
tous tissue interrupt the continuity of the collenchyma and break it up 
into bundles, the size of which depends upon the amount of schlerenchy- 
matous tissue which develops. 
This same general structure was shown by Sappin-Trouffy (47, p. 103) 
for oat stems and is figured for the sheath of wheat by Eriksson and 
Henning (12, pi. 4, fig. 40 b). 
Photographs of sections of the upper peduncle of neck of four varieties 
of wheat—Kota, C. I. 5878; Little Club, C. I. 4066; Marquis, C. I. 3641; 
and Sonem emmer, C. I. 4402—are shown in Plate 2, C, D, E, and F. 
It is evident that the relative proportion of sclerenchyma to collenchyma 
is not the same in each variety. In Kota the sclerenchyma divides the 
collenchyma into distinct areas, while in Little Club the sclerenchyma is 
much less conspicuous and the collenchyma is practically continuous. 
In the Marquis stem there are somewhat more sclerenchyma fibers than 
in the Little Club stem and there also is a greater tendency for union 
of the collenchyma areas than in the stem of Kota. In the stem of 
Sonem emmer a large amount of sclerenchyma always is developed. 
The morphological difference which may be expected in stems of 
different wheat varieties is illustrated in figure 1. The illustrations are 
actual tracings of permanently mounted sections prepared in the course 
of these experiments. In order to secure these tracings, the slides bearing 
the sections were placed on the stage of the microscope attachment of a 
projection lantern. The sections were then projected on white cardboard, 
placed 6 feet from the lantern. This greatly enlarged the sections, so 
that the details of structure could easily be traced with a pencil. The 
epidermis and fibrovascular bundles, together with all sclerenchyma, 
then were filled in with India ink, following which tracings of the different 
varieties were placed on a single large card and photographed. The 
interesting fact is the marked difference in the extent and distribution 
of the collenchyma areas in the different varieties. Figure 1, A, is made 
from a tracing of a section of Sonem emmer, C. I. 4402. In three years’ 
observation this variety has shown less infection from both P. triticina 
74025—24-5 
