Sexual fusions and spore development of the flax rust 
FRED D. FRoMME 
(WITH PLATES 8 AND 9) 
The variations found in different species of rusts so far investi- 
gated as to their sexuality, have made evident the desirability of 
further investigating a large number of forms, if we are to arrive 
at a definite understanding of the sexual processes in the Uredineae 
in general. From this standpoint I have taken up the study of 
_ the flax rust, Melampsora Lini (Pers.) Desm. The caeoma type 
of aecidium has in all cases proved favorable for the study of 
sexual fusions. The flax rust is no exception in this respect, and 
_ the abundance of the fusion stages in my material leaves little 
to be desired. Dr. E. W. Olive suggested the problem to me, and 
the results here recorded were largely worked out under his direc- 
tion at Brookings, S. Dak. 
A summary of the literature prior to 1908 has been.given by 
Olive (08) and still more recently by Maire (11), so that the later 
Papers only will be discussed here. While the sexual nature of 
the fusions in the rusts may be regarded as definitely established, 
the mor Phological character of the two cells involved and the 
Phylogenetic significance of the process are still points of con- 
tention. Two more or less distinct types of fusion: have been 
Fecognized. (1) A “partial cell fusion’? where fertilization is 
effected by the migration of the nucleus of a vegetative cell into 
4 special ‘‘fertile cell,”” as found by Blackman in Phragmidium 
violaceum. (2) A complete cell fusion between equal gametes, as 
figured by Christman in Phragmidium speciosum and suggested 
for Uromyces Caladii and Caeoma nitens. This type of fusion was 
later substantiated by Blackman in M elampsora Rostrupi, and 
by Olive in 7. riphragmium Ulmariae and Gymnoconia interstitialis 
(Cacoma nitens), Both of these processes Blackman considers as 
reduced types of fertilization that have supplanted a true ferti- 
lization, in which the spermatia functioned as male cells and 
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