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181)
SVEN ALGÉUS
colour reaction ön Ihat acid. was elaborated. Indole-acetic acid was
oxidized by the oxygen of the air with formation of yellow to
yellowish-brown products. The oxidation is cätalysed by light and proceeds at a
möre rapid rate in acid than in neutral solution. In darkness and neutral
solution the acid is ståhle, at anv rate for a short lime.
12. The oxidation products of indole-acetic acid exercise an
inhi-biting effect ön growth. Reduced illumination and short experiment time
counteract the disturbing influence. A determination of growth at an
unsuitable moment may easily convev the impression that the acid is
without effect or even has an inhibitive action.
13. Indole-acetic acid is superior to the homologous Compounds in
efficiency. Its effect is specific, and it cannot serve either as a source
of carbon or of nitrogen.
14. In the case of heterotrophic nutrition the stimulatory influence
of indole-acetic acid fails to appear.
15. Indole-acetic acid does not affect the size of cells except in
solutions of higli concentration. wliere cell division is stronglv inhibited.
16. A discussion of the experimental results leads to the view that
indole-acetic acid has other effects ön Chlorophyceae than it has ön
higher plants. Presumably it causes cell divisions by directly intervening
in the metabolism of the plasma.
17. 1-ascorbic acid exercised a positive influence ön cell division
in all the Chlorophyceae investigated. Its effect was observable only at
a rather high concentration, 1 g per 1. bower concentrations did not
give an increase of growth. The number of cells in the ascorbic acid
cultures was about double that in the controls. sometimes considerably
higher.
18. The effect of ascorbic acid is specific, and the acid cannot
serve as a source of carbon.
19. An investigation of the stability of ascorbic acid resulted in
disclosing why it is active onlv in high concentration, the ascorbic
acid heilig subject to rapid oxidation in dilule solution. Only in the
concentration 1 g per 1. does il last so long Ihat its physiological activity
cail be expected to make itself felt.
20. Düring the oxidation of ascorbic acid. a pH decrease takes
place in the originallv neutral solution owing to the formation of strong
organic acids. In a buffered solution (1/ls0 mol phosphate) the pH
decrease is, at least initially, insignificant.
21. Growth in an ascorbic acid culture ceases after 10—12 days
owing to the above-mentioned pH decrease and to the accumulation of
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