Experiment Station Record, Volume 92, January-June, 1945 Page: 224

                                
224                   EXPERIMENT STATION kECORD                     [Vol. 92
the protoplasm of the host cells becomes modified and a gummy material oozes out
and fills the intercellular spaces; at the same time, a reddish dyelike substance is
produced and absorbed by the cell walls. Mycelium advances but slowly into the
red zone, which forms quickly in resistant and more slowly in susceptible varieties.
In stalks where the cells are not functioning normally the mycelium spreads more
readily than in standing cane. In both stalk and leaf the mycelium penetrates the
cell walls through pits. Infection threads from appressoria were observed in epidermal
cell walls of the leaf sheath. Acervuli develop from mycelium packed in
the epidermal and subepidermal layers; this mycelium sends very small threads
through the walls of the epidermal cells. The conidiophores and setae develop from
these threads. The acervuli are subcuticular.
Life history of Cercospora on sweetclover, F. R. JONES. (U. S. D. A. coop.
Wis. Expt. Sta.). (Mycologia, 36 (1944), No. 5, pp. 518-525).--The life history
of C. davisii on Melilotus spp. has apparently been completed with the finding of the
spermogonial and ascigerous stages. Development of the former seems to depend
on temperatures below 20 C. The ascigerous stage-found on overwintered stemsis
described as Mycosphaerella davisii n. sp. Evidence of resistance to stem blackening
by this fungus is recorded.
A colorimetric method for the quantitative determination of minute amounts
of tobacco-mosaic virus and for the differentiation between some of its strains,
M. E. LOJKIN and H. P. BEALE (Contrib. Boyce Thompson Inst., 13 (1944), No.
7, pp. 337-354, illus. 6).-Quantitative microdeterminations of minute amounts of
this virus were performed with the aid of a Klett-Summerson photoelectric
colorimeter using the Folin phenol reagent, with the total weight of the virus required
being 0.006 to about 0.600 mg. The relation between virus concentration and colorimetric
scale readings cannot be expressed entirely satisfactorily by a single regression
line-either arithmic or logarithmic-calculated for data covering the whole range
of concentrations used. This relation can be best expressed for concentrations of
about 0.008-0.060 mg. per cubic centimeter of reaction mixture by a logarithmic
regression line calculated for this range and for concentrations below 0.008 mg.
per cubic centimeter, either by a different logarithmic regression line or by an arithnic
regression line fitting these lower concentrations. The photoelectric colorimeter
can be calibrated by rapidly determining in duplicate the colorimetric readings of
only a few solutions of known concentrations, plotting the results on logarithmic
paper, drawing two straight lines, one for each of the two ranges of concentration,
and using these two logarithmic straight lines as calibration lines. Thus the customary
procedure of making calibration curves can be eliminated.
Tyrosine, tryptophan, and cysteine-the three amino acids generally considered
responsible for the color developed by proteins with the Folin reagent-in the case
of this virus contributed not more than one-third of the colorimetric value given
by this protein with the phenol reagent in the presence of copper. In the absence
of more than a trace of Cu ions the colorimetric values produced by this virus with
the Folin reagent were much lower than the theoretical values calculated on the
basis of the tyrosine, tryptophan, and cysteine content of the virus. The colorimetric
values obtained for the ribgrass strain were higher than those of the type strain of
tobacco mosaic virus. This difference was found to be statistically significant when
the reaction was allowed to proceed in the presence of not more than mere traces of
Cu ions; it was also significant when too large an excess of the Folin reagent was
avoided. The colorimetric method is thus suitable for differentiating between two
strains of tobacco mosaic virus one of which is known to differ from the other in
its tyrosine and tryptophan content. There was no significant difference between the