138 Journal of Agricultural Research voi. xv, no. 3 



(i) In one type the embryo is dormant (incapable of growth when it is 

 naked and furnished ordinary germination conditions) and must go through 

 some very fundamental changes preliminary to growth. The changes 

 generally require considerable time — weeks or months. Some experiments 

 already published (i^, ip,2oy and many unpublished experiments made at 

 the Hull Botanical Laboratory by Mr, W, E. Davis and by Mr. R. C. Rose, 

 together with experiments reported in this paper, show the optimum 

 condition for after-ripening of dormant embryos, as well as some of the 

 changes that occur in the embryo during after-ripening. The seeds of 

 this type that have been studied in some detail are: Various species of 

 Crataegus, peach (Amygdalus persica), Ambrosia trifida, A. artemisiaefolia, 

 and basswood {Tilia americana) . The optimum conditions for the changes 

 seem to be a temperature of about 5° C, with a good oxygen and water 

 supply. The removal of the carpels or other coat structures hastens the 

 process markedly, and it does so, in part at least, by increasing the water 

 and oxygen supply. The following changes have been noted as after- 

 ripening progresses: Continuous rise in the vigor of the resulting seed- 

 ling, as manifested by rate of growth and resistance to fungal attack; 

 increase in the amount of water absorbed; increase in total acid; slight 

 (due to buffers) but evident increase of H"*" concentration; increase in 

 catalase activity; and increase in oxidase activity, at least as manifested 

 by action on guaiacum and on chromogens already in the plant. 



(2) In the second type of dormancy inclosing structures prevent 

 growth of the embryo by holding out some growth factor or holding in 

 some growth inhibitor. In hard seeds water is entirely excluded. In 

 Amaranthus and Alisma (13, 16) an initial rapid water absorption ceases 

 before the imbibitional and osmotic forces of the embryo are satisfied, 

 because the swelling of the seed contents is not sufficient to break the 

 seed coats. In many seeds the coats reduce the oxygen supply below 

 the necessary minimum for germination (75). 



If freshly harvested seeds of Johnson grass are kept in a germinator at 

 20° C, they will remain dormant for more than a year and probably for 

 many years. Under this condition they go through a change by which 

 they become less capable of germination at higher constant tempera- 

 tures (25° or 30° C.) than fresh seeds and very much less so than dry 

 stored seeds of the same collection. The senior writer (15) has spoken 

 of this deepened dormancy produced in a germinator furnishing con- 

 ditions unfavorable for germination as "second dormancy" and has 

 pointed out its rather general occurrence as reported in the literature. 

 Johnson grass seeds in dry storage gradually after-ripen, so that their 

 germination improves both at alternate and constant temperatures. 



The dormancy in seeds of Johnson grass is imposed by structures 

 inclosing the embryo (scales, pericarp, and tegmen), for removal of 



1 Reference is made by number (italic) to " Literature cited," p. 172-174. 



