1052 
■Journal of Agricultural Research. 
Vol. XXVIII, No. 10 
NUCLEI IN PROMYCELIA 
In Plates 3 and 4 are shown promycelia in various stages of growth from the 
germination of spores on to the formation of sporidia. The preparations studied 
were fixed and stained. The shaded areas merely represent living substance, 
cytoplasm and nuclei. The magnification is the same as used in Plate 2, which is 
somewhat higher than used in Plate 5, with which the figures should be compared. 
In Plate 3, A to K, are represented promycelia from spores of the rust on plant 
No. 497. At A and B division of the original nucleus has taken place; at C, the 
original nucleus has moved out into the promycelium where it will divide only 
once. The first cross wall is sometimes laid down before the first division (D), 
but most frequently division occurs in the spore itself. Any departure from 
standardized behavior on the part of the nuclei is reflected at once in the promy¬ 
celium (J, K). In one case the promycelium has only two cells, but one of them 
has two nuclei. In the one shown at K there are three cells, with one, two, and 
three nuclei, respectively. 
The other promycelia (PI. 3, L to R), are from spores in aecidia of strains 
which produce spermogonia. It should be noted that spores which develop 
promycelia with three or more cells are correspondingly larger. At M is shown 
a promycelium with six cells, but only four of them are alive and nucleated, while 
at Q there is shown a 4-celled promycelium, one cell of which is dead, the other 
three contain two nuclei each. At N is shown a sporidium with three nuclei; one 
cell of the prdmycelium was dead. Perhaps its nucleus migrated into the upper 
cell and entered its sporidium. A binucleated sporidium arises as the result of 
division of its original nucleus, generally after the sporidium is matured. 
Further details in the growth of promycelia are shown in Plate 4, A to J, from 
plant No. 489, where most of the aecidiospores are uninucleated. At A, the 
germ tube is put out before nuclear division, while nuclear division has already 
occurred in the spore shown at B. At C, the two daughter nuclei are passing out 
of the spore. Nuclear division is shown taking place in the promycelium at D. 
In cultures of this rust from plant No. 489 a great majority of the spores germinate 
in the forms shown from A to K. Figures L to S show types of germination from 
spores w hich had originally two or more nuclei. Three nuclei w 7 ere clearly visible 
in the large spore shown at L. One nucleus in each of the cells of promycelia 
shown at M, Q, and O is degenerating. At N, two nuclei are entering the 
sterigma. The central cell of the 3-celled promycelium at S has two nuclei, the 
others, one each. Figures T to V in Plate 4 show the prevalent type of pro¬ 
mycelium from the rust on plant No. 493. At X and Y are shown 4-celled pro¬ 
mycelia, but it seems two cells in each case have captured the nuclei, three and 
three at X, two and four at Y. If the terms “normal” and “abnormal” must be 
used in describing promycelia, it should be done for convenience only, meaning by 
“normal” the most common or prevailing type. All of the odd forms could 
probably be looked upon as regular if one only knew r the whole story. The 
w-riter has found aecidiospores with as many as seven nuclei in a young sorus 
whose spores were most regularly uninucleated. 
DISCUSSION 
The short-cycled orange-rusts of Rubus have much in common with those 
included under the genus Endophyllum. According to Hoffman (4), whose work 
has been confirmed by Moreau {12) } cell fusions occur at the origin of the aecidio- 
spore chains in E. sempervivi. The two nuclei resulting from the conjugate 
divisions of the nuclei contributed by the fusing cells, being cut off in the spore 
cell, fuse before the spore is shed. The fusion nucleus, which is much larger than 
either of the original nuclei, then undergoes a division so that the mature spore 
