Aug. 16. 1920 Further Data on the Orange-Rusts of Rubus 507 



per cent of the specimens studied. By this method it is possible to 

 identify with a fair degree of accuracy orange-rust specimens in herbaria 

 long after the spores are dead and have lost their color. It must be 

 remembered, however, that it is always necessary to have a liberal 

 quantity of mature spores in order to make determinations of value. 



The figures on Plate 93 show the variation in the size and shape of 

 spores from different collections of the short-cycled rust. The spores 

 shown in most of the figures are relatively small and angular. Those 

 shown in figures 9, 13, 39, 45, and 55 are large and round. They look 

 like the spores of the long-cycled rust, but their color and manner of 

 germination prove that they belong to the short-cycled rust. Figures 

 3, 10, 28, 29, 33, 35, 36, 44, and 57 show spores that resemble somewhat 

 those of the long-cycled rust. The specimens from which these aecio- 

 spores were taken can not be satisfactorily identified on the basis of 

 spore characters. The spores shown in all of the other figures on Plate 

 93 are characteristic for the short-cycled rust; but even in such cases 

 one can not be absolutely sure that they belong to this fungus, for 

 occasionally a specimen of the long-cycled rust bears spores like those 

 shown in figure 35 of Plate 94. The aeciospores shown in this figure 

 are small and angular; they do not look like spores of the long-cycled 

 rust. Spores shown in figures 4, 6, 8, 9, 19, 25, 42, and 43 of Plate 94 

 resemble to a certain degree spores of the short-cycled rust. On the 

 whole, however, aeciospores from different samples of the long-cycled 

 rust are more uniform as regards size and shape than are those of the 

 short-cycled rust. 



GENETIC RELATIONSHIP BETWEEN THE TWO ORANGE-RUSTS 



Although the two orange-rusts differ from each other in several char- 

 acters, it must not be denied that they are alike in many respects. They 

 are both systemic on species of Rubus. Their caeomas look much 

 alike, and in many specimens the aeciospores are quite similar. These 

 points of resemblance suggest a genetic relationship. Along with the 

 further evidence that the two rusts are distinct and different from each 

 other have come certain facts that strengthen this suggestion. In an 

 earlier paper the writer mentioned finding a promycelium in a culture 

 of aeciospores of the long-cycled rust. It was thought at the time 

 that the spore producing the promycelium might have entered as a 

 contamination. During the spring of 1917 and 1918 aeciospores of 

 Gymnoconia collected in different parts of the country were germinated 

 in great numbers. Each culture was carefully examined under the 

 microscope. In many cultures only germ tubes could be found. A 

 few promycelial germinations have been obtained, however, from spores 

 of every collection of Gymnoconia which the writer made during the 

 last two seasons. Sometimes such germinations are exceedingly rare, 



