BOTANY: J. ROSENBAUM 
161 
parable than a large mass of individual measurements. The con- 
stants obtained are the mean, median, mode, standard deviation and 
skewness. The measurements were made at the time of the first ap- 
pearance of conidia and oospores, at which stage the cultures most 
nearly approach the normal. The cultures were kept at approxi- 
mately 20°C. 
After making a number of measurements of conidia and observing 
the great variation in size, it occurred to me that by continual plant- 
ings from large selected individuals cultures could finally be obtained 
which would produce only large conidia and vice versa. After making 
a large number of dilution plates and selecting single large spores for 
five generations, it was found that there was no material difference 
between the percentage of large and small conidia in the original 
culture and the culture resulting from the last selection. 
The following summarizes the results of the above studies 
1. The various forms reacted differently on different media as re- 
gards rate of growth and spore forms produced. 
2. The temperature at which cultures are grown is a factor in the 
production of normal and comparable cultures. 
3. As a minor character, the macroscopic appearance of the growth 
on a given medium is of some value. 
4. The time of appearance of the spore forms from different strains 
of the same species on a given medium may vary, but eventually the 
same forms appear. 
5. On oat agar the mycelia of the various species cannot be distin- 
guished with any degree of certainty; on potato agar P. syringae can 
be distinguished from the remaining species by the fact that it pro- 
duces characteristic tuberculate mycelia. P. nicotianae can likewise 
be distinguished to a certain extent by the more gnarled mycelia and 
greater abundance of globoid particles of a fatty or glycogen nature 
within the threads. 
6. Measurement of the conidia can be employed to aid in delineating 
species, provided a sufficiently large number are measured. It is sug- 
gested that at least 200 should be measured and the different measure- 
ments grouped into classes. 
7. For the purpose of obtaining a quantitive measure of the shape 
of the conidia the ratio of the length to the width of the conidia should 
be obtained and the ratios likewise grouped into classes. A compari- 
son of the conidia of P. parasitica and P. nicotianae illustrates this 
point. Heretofore the differences in shape in the conidia of these two 
species would be expressed only qualitatively, calling the former 
