Dec. 22,1923 
Cytology of Wheat Stem Rust 
595 
The varying intensity of the reaction on the different hosts (Table V) 
when tested with a given form of the rust can be due only to inherent 
differences in the hosts. Brown (9) found that the enzym of Botrytis 
(pectinase) referred to above .attacked the cell walls of various higher 
plants, but was quite unable to affect the walls of the mosses and hepa- 
tics tested. This was ascribed to differences in the composition of the cell 
walls. It may be that lesser chemical differences in wall materials, such 
as might occur in nearly related varieties of plants, could still be great 
enough to cause differences in the degree of the reaction when exposed to 
the enzym. 
Smith (.54) observed an alteration of the host cell walls caused by 
Erysipheae that was very similar to the one found in rust attacks: 
The cell wall around the point of penetration (of the epidermis) is more or less altered 
and dissolved. Seen from the outer surface of Poa (Erysiphe communis ) and Kupato- 
nnm {Efysiphe cic ho race arum), there is an area surrounding the point of penetration 
which is entirely colorless, clear, and shining. The remaining portions of the epider¬ 
mal wall stain with safranin. 
In both of the cases cited, as well as in those of Colletoirichum lindemu- 
thianum (Sacc. and Magn.) Scribn., studied by Dey (jo), and Sclerotinia 
libertiana Fuckel, Boyle (<?), and many others, the fungus enters through 
the epidermal wall, and this reaction assists its entrance by softening or 
dissolving the cellulose layers below the cuticle. In rusts, on the contrary 
the entrance is through the stoma. 
In this connection it should be recalled that, although the germinating 
urediniospore and aeciospore of rusts enter only through stomata, the 
sporidium of many rusts, including Puccinia graminis Pers., penetrates 
the epidermal wall directly. De Bary says (4, p. 26 ): 
Eetztere (i. e. sporidia of Puccinia graminis) hatten in Masse gekeimt und iiberall 
sail man eine Menge von Keimschlauchen die Wand der Epidermiszellen durchboren 
und ins Innere dieser eindringen. 
In 1921, Waterhouse (45) also studied the entrance of the sporidium 
of Puccinia graminis tritici Erikss. and Henn. The sporidium produces 
a short germ tube which becomes attached to the epidermis and then 
pushes a very fine proboscis-like tube through the cell wall. 
No alteration in the nuclei when these were present in the epidermal cells or in the 
cellulose layers underlying the cuticle could in any case be detected at this stage. 
The sporidium apparently pierces the epidermal cell wall in much the 
same fashion that the haustorium mother cell pierces a mesophyll cell 
wall in making a haustorium. No evidence is given of any alteration of 
cell walls due to secretions, although such secretions would assist the 
entrance of the fungus here. This makes it more surprising to find such 
secretions at another part of its life cycle, where the natural openings 
of the plant are utilized for entrance. 
The utility of this alteration of guard cells, so far as the entrance of 
the fungus is concerned, is open to question. As already noted, the 
percentage of entries may be quite as great where the reaction is least 
as where it is greatest. Of course, the possibility is not excluded that 
the first stages of this reaction may affect the entry, although one can 
not tell from the present data whether the result would be to help or to 
hinder. It is conceivable that the first effect might be a stimulation of 
the guard cells which would result in their opening. The first effect, 
however, might be a loss of elasticity of the guard-cell walls that would 
result in their inability to arch out, in which case the stoma would re- 
