494 SECTIONAL TRANSACTIONS.—K. 
(4) Infection by insects would appear to be an insufficient explanation for 
the spread of mosaic disease and roll. 
(5) Seedlings isolated in soil not hitherto used for potato culture remained 
during 1923 free from mosaic disease or roll. 
(6) Susceptibility to mosaic disease is inherited; its degree is variable, and 
possibly the result. of several genetic factors. Susceptibility to roll is also 
inherited apart from that to mosaic. 
(7) Susceptibility is not necessarily linked with a high mortality, but lack 
of vigour will allow of intensification of the lesion after infection. 
(8) There is a definite relation between earliness and mortality amongst 
seedlings. 
31. Dr. D. Extis.—Sulphur Bacteria. 
32. Dr. T. L. Pranxerp.—The Ontogeny of Gravitational Irritability 
in Osmunda regalis. 
The author’s work on ferns has shown that ferns are sensitive to gravity— 
i.e., that the frond perceives the pull of the earth. If displaced from its 
normal upright position, the young frond makes a twist to regain it, though 
the possibility of its doing so was doubted by Charles Darwin. 
The sensitivity of fern fronds to gravity has been measured for the first 
time and shown to vary with various factors, the most important of which is 
the stage of development. A very young frond is far less sensitive than one 
unfolding its leaflets. It has also been shown that this sensibility is not 
present in the very young plant, but, though unseen, gradually rises as the 
plant develops, and to a much greater degree than the growth in length which 
can be seen. Plants differ greatly in their sensitivity to gravity—e.g., the Royal 
Fern has been shown to be sixty times as sensitive as Asplenium sp. It is 
probably the most sensitive vascular plant known in this connection. 
In the afternoon an excursion took place to the West Lancashire 
Sand Dunes. ° 
Wednesday, September 19. 
33. Discussion on The Effect of Soil Sourness on Plants. 
(2) Mr. A. G. Tanstey, F.R.S.—Soil ‘ Sourness’ as an Eco- 
logical Factor. 
Vegetation as an ‘ integration’ of climatic and soil factors. The task of 
ecology proper. The water factor. ‘Sour’ soil vegetation. Its importance in 
the British Isles. Relation to xerophilous structure. The theory of ‘ physio- 
logical dryness.’ Inadequacy of this theory. Relation of roots to basic ions. 
Adsorption of basic ions by colloids. Saturated and unsaturated humus. 
Sourness (acidity) in the chemical sense. Hydrogen-ion concentration. 
Correlation with vegetation. Exceptions. Theory of ‘buffer action.’ Direct 
effect of hydrogen-ion concentration on roots, and on the soil flora and fauna. 
The theory of ‘basic ratios.’ Interpretation in terms of reaction of different 
basic ions with the tissues. Specific effect of basic lime. Hydroxyl-ion con- 
centration. Other factors. Carbon-dioxide concentration. Badly aerated soils. 
Poverty in oxygen. Soil ‘sourness’ probably a complex phenomenon. Cor- 
relation of factors in nature. The disentangling of the various factors. 
(b) Dr. BE. J. Satispury.—Plant Distribution in Relation to 
Acidity. 
The occurrence of particular species in relation to the hydrogen-ion con- 
centration as exemplified by Mercurialis perennis, Pteris aquilina, &. The 
incidence of wild species with respect to this factor can be presented as variation 
curves in which an ‘optimum’ is clearly recognisable. ‘The effect of water 
supply in ameliorating acidity of the soil as shown in nature and the diminished 
effect of acidity in water cultures. ; ; 
The inadequacy of the ‘ basic ratio hypothesis’ as a general explanation of 
the facts of distribution, and the experimental demonstration that change in 
this ratio does not bring about vegetation changes in the direction postulated. 
