SECTIONAL TRANSACTIONS.—K. 555 
such problems as those of the syngonous condition (epigyny) and the nature 
of the gynecial members is indicated. Reference is made to the floral 
development of some of the species. The paper constitutes a pre- 
liminary account of an investigation of the floral structure throughout the 
Campanulacez. 
AFTERNOON, 
Exposition of exhibits in the laboratories of the Department of Biology, 
University College, Leicester (2.15). 
Tuesday, September 12. 
Joint Discussion with Sections D (Zoology, g.v.) and I (Physiology) 
on Genetics (10.0). 
Alternative programme for Members not attending the Genetics Dis- 
cussion :— 
Prof. F. A. F. C. Went.—Recent progress in the study of growth-substance 
(Auxin) in plants (10.0). 
Although growth substances in plants (auxins) have been studied else- 
where, the present paper is limited to an account of the work done in the 
Botanical Laboratory at Utrecht. As far as the influence of auxin on the 
cell wall is concerned, Heyn is continuing the studies he began at Paris 
and Leeds (i.e. X-ray investigations of cell walls), which led him to dis- 
tinguish between plastic and elastic extension. The first of these is due 
to the action of growth substance and is irreversible. "The elastic extension 
can be explained by a relaxation of the outer layers of the cell wall, due to 
the influence of strain. By studying the load extension relationship of 
the wall he was able to provide an explanation of differences of elastic 
extensibility, which occur as a result of changes of the rate of elongation. 
A study of phototropism with seedlings of Raphanus and Lepidium made 
by Van Overbeek has given results by which a synthesis of the theories of 
Blaauw and F. W. Went could be obtained. Here the auxin is produced 
in the cotyledons under the influence of light, and it flows towards the base 
in a longitudinal direction so long as the seedlings are kept in the dark or 
are equally illuminated on all sides. If, however, unilateral illumination 
is used, the flow of auxin is diverted laterally, so that the shaded side gets 
more growth substance than the other one and consequently grows faster. 
A curyature therefore results. 
On the other hand, light has a very distinct retarding influence on the 
growth of these seedlings. It may be proved that this—which is obviously 
the light-growth reaction of Blaauw—is due to a diminished sensibility 
of the cells to growth substance. The same quantity of auxin with the same 
cell gives a greater elongation in the dark than in the light ; whether this 
is a direct action of radiation on the cell walls or not has yet to be seen. 
Lateral illumination thus has two different effects on the seedlings of 
Raphanus sativus—(1) it diverts the growth substance to the shaded side, 
in consequence of which this side will grow faster ; (2) it diminishes the 
reaction of the illuminated cells to growth substance, in consequence of 
which the illuminated side will show a lesser reaction. Both effects are 
additive, and the result will be a positive phototropic curvature, the amount 
of which could be calculated by Van Overbeek. A somewhat similar 
