109 



The preparation for the germination of the dormant seed, according to 

 this theory, is the loosening undergone by the shell of the seed, as the result 

 of its swelling with water. The broken cell layers which have become per- 

 meable for gases now permit their rapid penetration and their condensation 

 therefore gives the first impetus for the process of oxidation which causes the 

 transformation of the reserve substances into diffusible forms. Since it can 

 be observed with the seed albumen of plants that the breaking down of the 

 starch in the seedling begins in the cotyledons in monocotyledons, it can be 

 assumed that the part richest in nitrogen, i. e. the embryonic tissue, under 

 the influence of oxygen will begin the metabolic reactions and by the develop- 

 ment of abundant enzymes act upon its surroundings. 



The disturbance in the second phase of germination can result only 

 from a lack of oxygen or also from an excess of carbon dioxid. The state- 

 ments of Th. de Saussure confirmed by Deherain and Landrin show that no 

 gas is so detrimental to germination as carbon dioxid. Seeds which are kept 

 in a mixture of oxygen and hydrogen germinate just as in atmospheric air; 

 yet an addition of a few hundredths of carbon dioxid to an atmosphere of 

 oxygen is enough to absolutely inhibit germination, when only the little roots 

 have appeared. If the amount of carbon dioxid is very considerable seeds 

 will not germinate. 



Carbon dioxid in excess is very injurious to other dormant parts of the 

 plant. Van Tieghem and Bonnier^ found in bulbs and tubers (Tuli- 

 pa, Oxalis crenata) which respired further in air containing a great deal of 

 oxygen, and therefore, produced carbon dioxid, that they formed alcohol in 

 an atmosphere of pure carbon dioxid. Tulip bulbs which had been kept for 

 a month in air free from oxygen were suffocated and remained without fur- 

 ther development. 



When seed has been sown too deep there is also an excess of carbon 

 dioxid and a lack of oxygen. The thick soil covering brings about injuries 

 and hinders the germination of the seed but can not, however, be expressed 

 in definite figures. Aside from the different requirements of the different 

 species, the optimum thickness of the covering differs for the same species 

 according to the composition of the soil, the amount and distribution of pre- 

 cipitation etc. On this account the results of the experiments often under- 

 taken to ascertain the best depth for sowing differ from one another as soon 

 as a definite statement of figures is undertaken. They all agree, however, 

 that in doubtful cases it is better to sow with too shallow a co^'ering than too 

 deep. 



The purpose of the coz'erlng is to hold the young seed firm and to retain 

 a sufficient degree of moisture. The shutting out of light comes less under 

 consideration. The retention of sufficient moisture for germination must be 

 primarily considered. If enough is present, the roots themselves will pene- 

 trate at once into the soil even when the seed lies superficially. On this ac- 



1 Bulletin de la societe botanique de France. Vol. XXVII, 1880, p. 83. cit. 

 V^^ollny's Forschung-en auf dem Gebiete der Agrikulturphysik. 



