74 GROWTH OF PLANTS 



seeds. The pressures used varied from 20 to 60,000 pounds per square 

 inch. Higher pressures softened the coats but injured the seeds. Lower 

 pressures softened the coats without injury. Small seeds required higher 

 pressure to soften the coats and also endured higher pressures without 

 injury. In the soil such factors as weathering, bacterial and fungal action, 

 and abrasion of agricultural instruments modify hardness. 



Several methods have been developed for overcoming hard-coatedness 

 in seeds before sowing. Soaking the seeds in hot or boiling water has been 

 long used. In the nineties Rostrup,^'"' a Swedish botanist, discovered that 

 the outside layers of the hard coats could be eaten away with concentrated 

 sulfuric acid followed by thorough washing to remove all acid. The 

 lengt,h of time for either of these treatments varies greatly with different 

 kinds of .seeds and to a degree with different crops of the same kind. This 

 is especially true of the sulfuric acid treatment. Many different sorts of 

 scarifying machines have been invented and used commercially. In such 

 machines the seeds are thrown against sandpaper, needle points, etc., to 

 scratch the hard outside surface of the seeds. Some of these have proved 

 useless, either because the impact broke the embryos or injury laid the 

 seeds open to infection. Porter and Brown ^^ have shown that shaking 

 hard, black locust seeds in a bottle for 20 minutes makes them water- 

 permeable. In small-seeded commercial leguminous seeds one will gener- 

 ally find a much smaller percentage of hard seeds in those threshed by a 

 mechanical huller than those hulled by hand. Even rubbing the seeds 

 through a sieve to get rid of the hulls softens some of the seeds. In short, 

 the threshing machine acts as a more or less effective scarifier. We have 

 discussed only a portion of the important data on factors that induce or 

 overcome water-impermeability of hard seeds in practice or in nature as well 

 as in storage and in the soil; but space does not permit a fuller discussion. 



Light as a Factor in Dormancy 



Some seeds require light for germination and many others are favored 

 by hght, while other seeds are completely or partially inhibited in their 

 germination by light. Light-favored seeds may remain dormant when 

 covered by soil to such a depth as to exclude the necessary light, or light- 

 inhibited seeds may fail to germinate if they are sown with little or no 

 cover. The first type of seeds should be sown on or near the surface of the 

 soil or otherwise treated to overcome the light need, and the second type 

 should be sown deep enough to prevent the inhibiting effect of light. 



Relatively little time can be given to this topic. Consequently, the 

 author will quote a summary of an article which he wrote in 1936 en- 

 titled "Effect of the visible spectrum upon the germination of seeds and 

 fruits" 2°' p-^2°~^22 and published in the two- volume treatise "Biological 

 effects of radiation."* This quotation will be supplemented by a brief 



* McGraw-Hill Book Co., Inc., New York, N. Y., 1936. Permission to quote this 

 material is gratefully acknowledged. 



