[newton] wheat stem RUST 201 



The sub-stomatal vesicle now sends out at one or more points 

 tube-like processes, the true infecting hyphae, into which the whole, 

 or a part, of the vesicular protoplasm passes (Plate IV, 9). Usually 

 these infection threads follow closely along under the epidermal cells, 

 and send small knob-like or flattened haustoria (suckers) into the 

 host cells (Plate IV, 10; Plate V, 12, 13, 14 and 15). It is by means 

 of these haustoria that the fungus obtains its nutriment. Occasionally 

 the hyphae strike straight across the sub-stomatal intercellular space 

 and branch between the mesophyll cells (Plate IV, 9). Not many 

 such cases were observed. When the infecting hypha forms a 

 haustorium in the first cell with which it establishes contact we say 

 that infection has taken place. 



The next stage in the development of the infection is the branching 

 of the hyphae between the cells of the leaf. This growth is accom- 

 panied, and indeed supported, by the sending out of many haustoria. 

 The hyphae continue to grow very rapidly from the third to the 

 seventh day, by which time they have usually attained their maximum 

 development. During this period two distinct kinds of branches are 

 seen, the short branches which ramify in the intercellular spaces 

 between the palisade cells, and the long, almost straight hyphae 

 which grow so quickly, and have such long segments, and so few 

 branches, that they remind one, to use Ward's simile, of "runners in 

 higher plants" (Plate V, 16). These runners are vacuolated but 

 rarely septate. They seem to be more in the nature of distributive 

 filaments. Haustoria are not developed by the quickly extending 

 runners, but are abundantly formed by the short branches which fill 

 the intercellular spaces between the cells. 



About the fifth day the hyphae branch very rapidly, and begin 

 to mass themselves in a dense weft beneath the epidermis, preparatory 

 to the formation of a pustule. The epidermal cells are wedged apart, 

 and by the eighth day the epidermis has been completely ruptured, 

 after which the spores are shed in great profusion. 



In the course of the developmental cycle just described the 

 fungus does not seem to spread very far from the point of infection. 

 Indeed, when large areas of the leaf are involved a number of points 

 of entry can nearly always be found. 



In the susceptible host there seems to be a ready adjustment 

 between host and parasite during the early stages of the disease. 

 In spite of the fact that the mycelium is growing vigorously the host 

 cells are not severely injured. Even in preparations of tissue 

 thoroughly infested for some days, in which the spores have burst 

 through the epidermis, the protoplast may retain its organization 



