630 Journal of Agricultural Research voi. xv, no. 12 



(PI. 58, A). After receiving its nucleus the spore is abstricted. No evi- 

 dence of a collar like that figured by Blackman (2) was seen, unless the 

 odd constriction of the cytoplasm shown in Plate 58, A, c, can be consid- 

 ered as a collar. This phenomenon is quite common. Occasionally long 

 hyphal filaments grow some distance out beyond the tips of the sporo- 

 phores. The structure of the spores is typical of pycniospores in general. 

 They appear to be completely nonfunctional. No attempt was made to 

 germinate them. 



The dark areas on the bark which indicate the location of the pycnia 

 are designated pycnial spots (PI. 54, A, h; 48, B). They are honey-yellow 

 to brown-yellow at first, but they gradually assume a color like that of 

 clotted blood as the pycniospores mature and ooze out, and may finally 

 become almost black. Later, when the covering of cork cells sloughs off 

 or is eaten off by insects, the drying cells of the host tissue beneath turn a 

 typical light pink color. After reaching maturity the pycnium is cut out 

 from the host tissue by the formation of a cork cambium and the deposition 

 of a cork layer at a depth of four to six cells below the bottom of the 

 pseudoparenchyma base of the sorus (Pi. 50, C, h). It is the exposure of 

 this layer which reveals the typical color of newly formed cork cells. 

 The pycnial spots, whether young or old, are valuable diagnostic char- 

 acters, because they often make possible the detection of infected trees 

 before secia are produced — that is, in time to destroy such trees before 

 geciospores can spread the disease. 



^CIA 



i^cia appear in April, May, and June. Very often the whole area on 

 which aecia can normally develop in a given season is covered with closely 

 crowded son pushing their way through widening cracks in the bark. 

 By the middle of May the peridia are usually broken and the spores 

 escape in orange- yellow pollen-like showers. Spore production continues 

 for some time after the aecia open. Young aecia are easily obtainable at 

 the edge of the aecial area where their presence is indicated by a thin 

 yellow hne just beneath the outennost layers of the bark cells. The 

 hyphae contributing to the formation of the secium are aggregated into a 

 mycelium, which is clearly made up of elements running tangentially 

 among the host cells at a depth of 6 to 10 cells below the periderm layer. 

 By their continued growth these hyphs force the host cells apart, so that 

 the latter become isolated and embedded in a matrix of tangled myce- 

 lium (Pi. 50, B). This separation may extend to a depth of 15 to 20 cells 

 in the cortex. About 6 to 8 cells below the periderm layer a mass of 

 pseudoparenchyma is formed by the packing together of the hyphae. In 

 the pseudoparenchyma a layer of fertile cells becomes discernible by their 

 denser protoplasmic content (Pi. 50, A, 6; 54, B, fc). 



The fertile cells cut off rows of sterile cells (PI. 54, B, i'c) , 6 to 1 2 cells long, 

 which may increase in size at first, but which later degenerate to make 



