Dec. 22, 1923 
Cytology of Wheat Stem Rust 
575 
cells, branching freely and forming numerous haustoria. Within a 
limited area in the leaf, perhaps a millimeter in diameter, each chink 
and cranny of intercellular space becomes filled with the fungus, which 
forms little masses of pseudoparenchyma conforming closely to the 
shape of the irregular passages they occupy. The host cells retain their 
shape and spatial relations, and in spite of the fact that the fungus is 
choking the air passages around them and forming numerous haustoria 
which absorb the food within them, these host cells are still living and 
fairly vigorous. Haustoria are formed in abundance and expand freely, 
growing into long vrorm-shaped bodies sometimes extending across the 
cell. 
When the fungus is well established in its host and is rich in proto¬ 
plasm and food, it enters upon a new phase marked by two activities, 
the formation of uredinia arid the sending out of long stolon-like hyphae 
to fresh areas in the leaf tissue. Both of these activities are aided by 
a translocation of food materials through the hyphae from the center of 
the mycelial mass to its periphery. The method by which this trans¬ 
portation takes place is not obvious. The hyphae are septate, being 
composed of relatively short binucleate cells. The septa are not porous, 
so far as could be determined, or if they are, the pores are ultramicro- 
scopic in size. Yet, in some fashion, the contents of the hyphae, and 
to some extent even those of the haustoria, move out to the regions of 
active growth. 
Haustoria in full vigor contain apparently dense granules scattered 
through the cytoplasm. In infections about a week old, the granules 
begin to disappear, then much of the cytoplasm follows, leaving the 
haustorium nearly transparent. Something very similar in appearance 
occurs in the hyphae of the whole central region of the infection. Food 
granules and cytoplasm disappear gradually, leaving the hyphae nearly 
empty. Plate 1, B, shows a bit of intercellular mycelium at b, and a 
haustorium at a, from an infection a week old. The haustorium is pale 
and its contents scant, and the hyphae b outside of the cell are nearly 
empty. Another change is in the haustorium mother cell, which hith¬ 
erto has been clear, empty, and thin-walled. Now (pi. 1, B, at c) the 
haustorium mother cell presents an appearance suggesting that the 
walls, or at least the inner lamellae of the walls, are swelling. The 
swollen wall, if such it is, takes a faint pink stain and is glassy in appear¬ 
ance. Phis change varies greatly in different lots of material, and, in 
extreme cases, as in plate 1, C, at a, and D, at a and b f the lumen of 
the haustorium mother cell may be almost obliterated. 
As the haustoria become transparent, due to the disappearance of the 
granules, there frequently are revealed within them one or two rounded 
bodies, strongly resembling nuclei. These may be the nuclei of the 
original cell which contributed its contents to form the haustorium. 
These are to be seen clearly in Plate 1, C, at b , in D at c and d , and in 
E at a. 
A minor part of the food drained from the central mycelium is util¬ 
ized by the stolon-like hyphae. Unlike the earlier hyphae, these “ stolons ” 
do not feel their way along the surface of the cells, conforming closely 
to their irregularities, nor do they form a haustorium whenever the tip 
strikes against a host cell. These well fed, rapidly growing, sparsely 
septate stolons strike out away from the center of infection, growing 
across the intercellular spaces as straight as the irregular passageways 
of the leaf permit. When they reach fresh tissues, they start new 
