100 University of California Publications in Botany \yo\.. 5 



ating ease and in the propagating house, showed a fairly regular 

 increase in total germination, beginning with the lightest grades 

 of seed and running up to the heaviest. 



The results of numerous investigations on the delayed ger- 

 mination of seeds and the water and oxygen requirements for 

 germination, point out the probable causes of the early ger- 

 mination of the light seed and the delayed germination of the 

 heav}^ seed (Shull, C. A., 1909 and 1911, and the literature there 

 cited). Thus (1) the seed-coverings of the majority of the light 

 seed may be more permeable to water and oxygen than those of 

 the hea\'y seed, or (2) the water and oxygen requirements for 

 the germination of the heavy and light seeds may not be the 

 same and, (3) the oxygen content of the medium surrounding 

 the seeds in the germinating case and in the propagating house 

 was certainly higher in the latter situation. Thus, again, with 

 a high permeability for water and oxygen in the seed coats and 

 a low oxygen requirement for the germination of the light seed, 

 we should find this light seed showing high average germina- 

 tion in the germinating case. On the other hand, with slight 

 permeability of the seed coats of the heavj' seeds for water and 

 oxygen and a high oxygen requirement for their germination, 

 we might expect to find the average germination in the ger- 

 minating case relatively low and that rapid increase in ger- 

 mination would sooner or later be apparent with the increased 

 oxygen about the seed in the propagating house. The conclu- 

 sions of various investigators (Strasburger, Noll, etc., p. 258) 

 that variations in temperature increase germination may also 

 give a suggestion as to the cause of the high average germina- 

 tion of the heavy seeds in the propagating house (see also Raci- 

 borski, 1900) . 



