122 FROMME: SEXUAL FUSIONS IN FLAX RUST 
ment of the gametes, as Olive finds for Triphragmium Ulmariae. 
In the early stages of the sorus the gametes are not always placed 
in an even parallel layer but may vary in elevation. The irregu- 
larities due to the loose indefinite nature of the caeoma quite fully 
account for the unequal position of the gametes. The presence 
or absence of sterile cells at the time of fusion, again does not 
seem a good criterion for determining the relative age of the two 
gametes. I have observed several cases where both gametes at 
the time of fusion still have two sterile cells intact. See FIG. 10. 
As I have pointed out, however, these sterile cells have usually 
disappeared at the time of fusion. 
The abundance of sexual fusions in Melampsora Lint is most 
striking. The sexual fusions figured by earlier students have been 
scattered and disconnected and apparently only occasionally 
found. In my material I have sections showing practically 
every pair of gametes in the sorus in some stage of fusion. These 
stages are so abundant that there can remain no doubt whatever 
that the binucleated condition in this form is always instituted 
by means of a cell fusion between entirely equivalent gametes. 
Fic. 11 shows a collection of “fusion cells’? that were especially 
well situated for drawing. A triple cell fusion is shown in the 
center of the group with three ordinary fusions. on either side. 
The “fusion cells’’ are rather old here, one or more aecidiospores 
having been produced in most cases, but the double ‘‘basal cells” 
still show the original fusions distinctly. Fic. 12 shows another 
series of cell fusions. The five pairs of gametes in this instance 
are of about the same age as those shown in FIG. 11. Younger 
pairs are shown in FIG. 13. The gametes on the left have just 
completed the fusion while those on the right have cut off the 
first aecidiospore mother cell. Fic. 14 shows a mature, binu- 
cleated aecidiospore with a chain of intercalary cells and young 
aecidiospores ending in a double cell fusion below. The process 
by which the chain of spores and intercalary cells are formed has 
been fully described and need not be taken up here. 
As previously mentioned, Olive has figured a somewhat differ- _ 
ent method of forming the “basal cell,” in Triphragmium Ulmariae, 
from that shown by Christman in Phragmidium Potentillae- 
canadensis and P. speciosum. As found by Christman, the 
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