116 Murphy.—The Morphology and Cytology of the 
study of their development and structure. This paper presents such a study 
of P. erythroseptica ; and while it would be unsafe to generalize too far in 
view of the wide variations found in the cytology of the nearly related genus 
Albugo , yet in the absence of further study it may be assumed that they are 
all alike in cytology and development as they are in mature structure. The 
work was undertaken in the first place to study the genesis of the sexual 
organs, but it was found that the nuclear phenomena could be advantageously 
followed at the same time, and so both studies were carried on side by side. 
On account of the greater difficulty experienced in the latter branch the 
work soon assumed an almost purely cytological aspect. 
Pethybridge s description of the sexual organs of P. erythroseptica may 
be summarized as follows: The hypha which is about to give rise to an 
oogonium or oogonial incept, grows up to and pierces the antheridium, grows 
through it and out at the other side, where it appears as a club-shaped 
structure and later as a spherical oogonium. He suggested as a possible 
explanation of this process that perhaps fertilization might take place while 
the oogonial incept was still within the antheridium. The emptying of the 
antheridium, the contraction of the protoplasm of the oogonium, and the 
formation of the oosphere were also described. These observations were all 
made on living material or on fresh preparations. Pethybridge was unable, 
owing to pressure of other work, to proceed further with the matter, and he 
very kindly handed over all his cultures to the writer, who had more time 
at his disposal. The results which have been obtained with stained sections 
corroborate Pethybridge’s observations in all particulars. It may be stated 
in general that no theory will fit all the facts but that given by him, and 
those which give even a very partial explanation are more radical and im¬ 
probable than the one they Seek to replace. 
The writer desires to express his indebtedness to Dr. G. H. Pethybridge 
for furnishing the necessary cultures ; to Professor Farmer, under whom the 
work was begun at the Imperial College of Science and Technology; to 
Professor Claussen, in whose laboratory it was continued; and finally to 
Professor Whetzel, who gave the writer every kindness and facility in the 
Phyto-Pathological Laboratories at Cornell University. 
Technique. 
The material used in this study was all derived from pure cultures of 
the Fungus grown on agar slants and in Petri dishes. At first clear extract- 
of-oat agar was used, for the preparation of which see Pethybridge and 
Murphy ( 30 , p. 578). The resulting medium is practically clear, and the 
stage, of development of the sexual organs can be determined under the 
microscope before fixing. It has no other advantage, however. The Fungus 
fruits so sparingly on it that a large amount of material has to be sectioned 
