SECTIONAL TRANSACTIONS.—M*. 523 
The combined data suggest that D. variabilis is a hybrid between two tetraploid 
species, one belonging to the ivory-magenta, and the other to the yellow-orange- 
searlet flower colour group. In the descent of the tetraploid progenitors of 
D. variabilis from a diploid ancestral stock differentiation occurred, and, among other 
distinctions, gave rise to the flower-colour groups. Following hybridisation between 
members of these groups, doubling of the chromosome number occurred, giving the 
fertile octoploid species D. variabilis. The garden dahlia, therefore, combines the 
results of specific differentiation with a high degree of polyploidy, i.e. qualitative with 
quantitative differences. 
Most seed-bearing polyploids are hybrid-polyploids with an origin and constitution 
more or less similar to that of D. variabilis. Since the majority of cultivated plants 
of economic importance are polyploids, it is desirable that we know their constitution 
and probable behaviour on breeding, and that appropriate methods are adopted for 
the breeding of these genetically complex types. 
Miss C. O’Connor and Dr. Repcuirre N. Saraman.—Recent Progress in 
Breeding Potato Varieties Resistant to Potato Blight (Phytophthora 
infestans). 
A rapid and fairly reliable method of testing the resistance of potato seedlings to 
Blight (Phytophthora infestans) has been developed, based on the method used by 
Miiller in Germany. 
The seedlings are grown in a closed glass case. It is shaded from direct sunlight, 
the relative humidity is kept at saturation and the temperature is controlled as far 
as possible. The seedlings are also sprayed with nicotine to prevent insects carrying 
virus diseases from living in the case. 
To test their resistance to Blight, seedlings are sprayed almost daily with a sus- 
pension of P. infestans conidia germinating in water, an atomiser being used for 
spraying. The most susceptible seedlings are killed by the fungus five to six days 
after the first spraying. More resistant seedlings withstand this treatment for two 
or three weeks, and the most resistant or immune seedlings cannot be infected. All 
the batches of seedlings to be tested are sprayed at least twenty times before the 
survivors are removed from the case, and selections of desirable types are made from 
them. Later in the season, and in the following years, further tests are made on 
offsets and cuttings which are again put into the case. 
Until 1928, no seedling derived from domestic stocks had been shown to be immune 
to Blight. Resistance that was claimed for some varieties such as Champion, has 
been shown to be purely physiological and due to late maturity. The senior author had 
found that the wild species, Solanum utile, was almost completely immune to Blight. 
He raised a large series of hybrids from the cross 8. utile x Domestic variety. From 
these and certain back crosses with domestic varieties we have obtained a few highly 
resistant seedlings endowed with moderately good domestic characters. In the last 
three years we have succeeded in finding a few seedlings of purely domestic parentage 
which are also highly resistant to Blight. They have only occurred in certain families 
and then only about 1 in 500 seedlings. 
The South American species S. antipovichi, Aya Papa, and Papa Amarilla have 
also been found to be practically immune to Blight. Seedlings of S. utile and S. 
antipovichi, selfed, are also 100 per cent. immune. So far, however, in the F, from 
erosses Resistant x Domestic, the great majority of the seedlings have been sus- 
ceptible, and the evidence tends to show that resistance to P. infestans is a recessive 
genetic character. Unfortunately, bud-dropping, infertility and lack of pollen in 
potato flowers, make it very difficult to carry out this breeding work systematically. 
Friday, September 25. 
Discussion on Vegetative Propagation :— 
Prof. J. H. Priestitey, D.S.0.—Polarity in Cutting Propagation. 
Isolated pieces of shoot or root of the flowering plant show very marked polarity 
inregeneration. This is particularly true of the Dicotyledon, in which the regenerative 
activities of the isolated piece seem to be closely connected with the cambial activity. 
When the cambial activity of the normal plant is studied, this is seen from its inception 
