PLANT PHYSIOLOGY 373 



on the ground during the winter months has yet to be found. 

 A. Lumiere has pubhshed a series of articles (" Le Reveil de 

 la Terre," Comptes Rendus de I' Acad, des Sc, 1920, p. 868 ; 

 " Action nocive des Feuilles Mortes sur la Germination," ibid., 

 192 1, p. 232 ; " Le Rythme saissonier et le Reveil de la Terre," 

 Rev. gen. de Bot., xxxiii, 1921, p. 545) dealing with the results 

 of an attempt to discover a controlling factor of this seasonal 

 rhythm in vegetation apart from any direct effect of such 

 obvious environmental factors as temperature, light, and 

 rainfall. In these three papers the author has shown con- 

 clusively that the soluble products of fallen leaves, debris of 

 annual plants, etc., act as powerful reducing agents and may 

 inhibit the germination of seeds, and has developed the interest- 

 ing hypothesis that these substances have an important role 

 in the seasonal periodicity of vegetation under natural condi- 

 tions. For the normal germination of most seeds a free supply 

 of oxygen is necessary. Hence it becomes evident that any 

 reducing substance which may be dissolved out of rotting 

 leaves and other decaying vegetation and remain in the 

 soil for a certain period will tend to absorb oxygen and deprive 

 the seeds of a supply sufficient for germination. Sooner or 

 later these soluble substances either become oxidised, a change 

 which is accelerated by the rising temperature of spring, or 

 are washed out of the superficial soil by rain. When either 

 result is achieved the seeds readily germinate. As evidence 

 in support of his theory Lumiere records the results of a 

 number of experiments, from which the following are a selection. 

 In one experiment leaves of various plants were soaked in 

 rain-water immediately after they had fallen and the extract 

 was employed in dilute solution to water the soil in which 

 barley grains had been sown. The germination of the seeds 

 was completely inhibited. 



In another experiment similar leaves were left for three 

 months freely exposed to the air, during which interval they 

 were acted upon by various micro-organisms, amongst which 

 a species of Bacillus was prominent. At the end of this period 

 the decaying vegetable matter was extracted with water. In 

 both cases the extract was found to possess powerful reducing 

 properties and to inhibit the germination of barley seeds. 



In a third experiment a sample of soil from open ground 

 was taken in November and divided into two similar lots. 

 One of these was repeatedly washed with distilled water in 

 order to dissolve out soluble substances, whereas the other was 

 simply kept moist with distilled water. After a few days at 

 the temperature of the laboratory seedling plants appeared on 

 the former, but not on the latter, thus indicating that the 

 repeated washing with distilled water had eliminated some 



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