78 GROWTH OF PLANTS 



Muenscher ^^ shows that Ught is necessary for the germination of Lobelia 

 inflata and that other factors will not substitute for light. These seeds 

 gave no germination when covered with 1 cm. of soil. L. cardinalis and 

 L. siphilitica seed also require light, while L. tenuior and five forms of 

 L. Erinus seed germinate equally well in light and dark. Funke *^ has 

 recently confirmed the findings of Wiesner and others on the role of light 

 in maintaining the life and inducing the germination of Viscum album 

 seed. The seeds are injured by two days of continuous dark. Most rapid 

 germination is produced by removing the endosperm and subjecting the 

 embryo to continuous illumination, artificial light at night, and sunlight 

 during the day. The failure to germinate in Belgium during the winter, 

 he feels, is due to low light intensity and daily duration rather than low 

 temperatures. 



Sprague ^'* further confirms the fact that dry storage overcomes the 

 need for light; Poa pratensis seed six months after harvest no longer 

 required light or alternating temperature for germination. Jensen ^^ sug- 

 gests that exposure of seeds to artificial light lengthens their viability in 

 dry storage. Various workers ^^' ^^' *^ have shown that certain light rays 

 modify enzyme content, metabolism, and growth substances in germinat- 

 ing seeds, but in no case does this work explain how light induces or hinders 

 germination. 



Oxygen Deficiency and Dormancy 



Growhig plants with their intracellular aeration systems connected with 

 stomates and lenticels are much better equipped to get the needed oxygen 

 supply from the air than are embryos of seeds which are, in the main, 

 completely sealed within seed coats and often additionally covered with 

 fruit coats and other structures. 



Using common cultivated species of plants (common bean, broad bean, 

 cress, savory, and Hydrangea) in a special growing chamber, Schaible ^"" 

 grew the plants and germinated the seeds in one-fourth of an atmosphere 

 of pressure with a continuous change of the atmosphere; in one case he 

 drew air through the chamber, thereby giving one-fourth the partial 

 oxygen pressure of a full atmosphere of air; and in the other he used oxygen- 

 enriched air so that the partial oxygen pressure in the chamber was equal 

 to that in a full atmosphere of air. Plants in the reduced pressure grew 

 much faster than plants in a full atmosphere, regardless of whether the 

 partial oxygen was normal or one-fourth normal. The atmospheric pres- 

 sure determined the rate. Seeds germinated a little better in the reduced 

 pressure of oxygen-enriched air than in a full atmosphere of air, but very 

 much worse in the reduced atmosphere without oxygen enrichment. In 

 other words, the oxygen content of the air is far above that needed for the 

 fastest growth of plants, but it is not so far above that needed for the 

 germination of seeds. The latter is conditioned by the slow passage of 

 oxygen through the seed and fruit coats. 



