DORMANCY IN SEEDS 



81 



after maturing. Both seeds absorb water readily, and neither the bur nor 

 the old dried ovary wall seems to play any considerable part in the delay. 

 Fig. 23 shows the bur, the two seeds, and the arrangement of the seeds 

 in the bur. Without evidence except that enzyme differences were used 

 to explain many plant responses, Arthur concluded that enzymes were 

 more abundant or developed faster in the lower seed, and consequently it 

 germinated more promptly. 



Figure 23. The cockle- 

 bur. Upper row: upper seed. 

 Middle row: intact bur, cros.s 

 section of bur showing the 

 two seeds, longitudinal sec- 

 tion of the bur showing the 

 two seeds. Lower row: lower 

 seed. 



Crocker ^^ attempted to explain this delay in the germination of the 

 upper seed of the cocklebur. He observed the following facts: Germina- 

 tion failure of the upper seed, with the coat intact and either in or out of 

 the bur, is due to the seed coat and not to the bur or ovary wall; this is 

 true in spite of the fact that the three-layered seed coat is very thin, about 

 0.034 mm at the cotyledon end and 0.145 mm at the radical end of the 

 seed. When the excised seeds or the seeds in the bur are placed in a germi- 

 nator in air at 22° C (71° F) the lower seed only germinates. When placed 

 in a germinator \\\ih a full atmosphere of oxygen at 22° C (71° F) both 

 seeds germinate, but the growth in the upper seed starts in the cotyledons 

 where the coat is thin. When placed in a germinator at 33° C (91° F) 

 with air, both seeds germinate. When the seed coats are removed, both 

 embryos germinate promptly, even at 18° or 20° C (64° or 68° F). Prick- 

 ing the coat of the upper seed with a pin causes it to grow in a germinator 

 at 20° C (68° F) in air, but the growth starts in the region of the prick. 



From these results Crocker ^^ concluded that both seeds would germinate 

 in the first season after harvest if the seed bed reached a temperature as 

 high as 33° C (91° F) at a time of adequate water and air supply; that 

 the failure of the upper seed to germinate at lower temperatures was due 

 to the fact that the thin seed coat reduces the supply of oxygen to the 

 embryo below the minimum needed for germination ; and that the oxygen 

 supply to the embryo of the lower seed was restricted by the coats, for 

 it too germinated at a somewhat lower temperature when the coat was 

 removed. Naturally, one inquires why the upper seed germinates during 

 the second or later seasons. Crocker suggested that the delicate semiper- 



