LEAF-SPOT OF ALFALFA AND RED CLOVER. 25 
but even after 52 hours the germ tubes had not advanced beyond the cell first 
penetrated. 
Melilotus alba by Pseudopeziza trifolii. — Three days from inoculation a few 
germ tubes were found distinctly inside epidermal cells, but they had advanced 
but slightly. Penetrated cells were not yellowed. 
Melilotus alba by Pseudopeziza medicaginis. — The cultures used for inocula- 
tion in this series did not produce many spores. Nevertheless, after four days 
two penerations were found. The penetrated cells were very slightly yellowed. 
Trifolium hybridum by Pseudopeziza trifolii. — No examination was made 
until two days after inoculation. At this time penetrations were abundant 
and easily seen. In a few cases germ tubes had advanced into cells adjoining 
those first penetrated. No yellowing of penetrated cells was observed. 
Trifolium hybridum by Pseudopeziza medicaginis. — In 24 hours penetrations 
were abundant, but the germ tubes had not advanced far into penetrated cells. 
Penetrated cells showed no yellowing. 
It appears that the results which are shown above are exactly 
parallel to the results obtained from inoculations. In every case 
where infection in varying degree has been attained, penetration has 
occurred abundantly and the growth of the hyphse within the host 
cells has been rapid without causing discoloration of the cell con- 
tents. In cases where visible infection has not been obtained, the 
relative number of penetrations is usually reduced, but in any case 
growth of the germ tube ceases promptly upon entering the epidermal 
cell. Thus, the resistance which the plants that can not be infected 
offer appears to be due not to any mechanical obstruction to entry, 
but to something within the epidermal cell which prohibits growth. 
These infection experiments supported by penetration studies have 
failed to produce a completely successful infection of any of the 
hosts tried except those from which the fungi were isolated. This 
result is very different from that which was anticipated from a 
consideration of the host lists. Have we here a group of closely 
similar fungi highly specialized in their host relationships? 
It is noteworthy that in both host lists only two or three of the 
species of Trifolium are native to America. All the rest have been 
introduced if they occur here at all. Moreover, some of these intro- 
duced hosts which are widely distributed do not appear to be 
attacked by this parasite except in certain limited areas. For ex- 
ample, Pseudopeziza medicaginis has been found on Medicago lupu- 
lina only in New York; P. trifolii has been reported on Trifolium 
repens only by McClatchie (1895) on the Pacific coast 1 and not at 
all on T. hybridum. It is not likely that these fungi have been over- 
looked on these hosts in other localities, and therefore the conclusion 
that they do not always pass to these hosts can hardly be escaped. 
Thus, there is reason to suspect that these two species, as now re- 
1 An excellent collection of Pseudopeziza on Trifolium repens now in the possession of 
the writer was made by Mr. C. W. Hungerford at Olga, Wash., on Sept. 3, 1916. 
