114 ARTHUR: IMPORTANCE OF THE SPERMOGONIUM 
Sappin-Trouffy regards the uninucleated condition as suggestive 
of senility. 
Waiving this inference, observe that both aecidiospores and ure- 
dospores are binucleated, and that the nuclei arise from the binucle- 
ated mycelium in the usual vegetative manner characteristic of the 
rusts. Both spore-forms are therefore conidia, and serve the purpose 
of rapid propagation of the species, being purely asexual in origin 
and function. This, of cqurse, has always been assumed for the 
uredospores, but often denied for the aecidiospores. The work of a 
number of eminent cytologists no longer leaves the slightest doubt 
regarding the correctness of this view, and whatever degree of 
sexuality has been assumed for the aecidium must henceforth be 
abandoned. 
Turning to the teleutospore we find that each cell contains two 
nuclei when young, but that they soon fuse in a manner highly 
suggestive of a sexual function. The explanation of this nuclear 
fusion as a genuine sexual act was first proposed by Dangeard 
and Sappin-Trouffy nearly ten years ago, and quite recently reaf- 
firmed by Harper, and again very recently by Holden and Harper 
in an important contribution to the nuclear phenomena in Co/eo- 
sporium. During germination the single nucleus of the teleuto- 
spore moves into the promycelium, and by dividing twice furnishes 
a nucleus for each of the four sporidia. In the sporidium the nu- 
cleus once more divides, but this time the process is unaccompan- 
ied by division of the cell, and thus the vegetative binucleated 
condition is again established, which is maintained throughout the 
life-cycle until the teleutospore is again reached, unless the spet- 
mogonium may be considered as introducing an interruption. 
However, as the uninucleated spermatia are discharged into the 
air and come to naught, and as the mycelium bearing the spermo- 
gonium continues the cycle by developing potentially active spores, 
aecidiospores for example, it is evidently permissible for our present 
purpose to ignore the spermogonium in tracing the nuclear cycle. 
Although our knowledge of the nuclear history is not com- 
plete, yet enough has been established to permit of a reasonable 
inference regarding the nuclear cycle, which may be stated in 
brief, as follows: A fusion takes place in the teleutospore, having 
the physiological effect of a sexual act, and establishing the begin- 
