48 GROWTH OF PLANTS 



seeds and below 30° C (86° F) for upper seeds. We shall describe more 

 fully the behavior of the two cocklebur seeds in the next chapter. To 

 throw the embryos of cockleburs into the dormant condition, the intact 

 seeds were placed in a germinator at a high temperature, 28° C (82° F) or 

 above, with greatly reduced oxygen pressure. The reduced oxygen pres- 

 sure can be obtained by displacing some of the oxygen in the air with a 

 gas such as nitrogen or hydrogen, or by imbedding the whole seed in agar 

 jelly or the radical end in modelling clay in a germinator as shown in 

 Fig. 14. With the intact seeds in a high-temperature germinator at re- 

 duced oxygen pressure the embryos become dormant in from two to several 

 months. Thornton ^^ confirms Davis' work on the cocklebur. The dor- 

 mant seeds can be thro\vn out of dormancy by placing the seeds in a germi- 

 nator at 5° C (41° F). While the intact seed in secondary dormancy will 

 not germinate, the embryo can be made to germinate if the seed coat is 

 removed, but the growth is very slow and sluggish and the seedling is 

 dwarfish mth abnormal leaves. Fig. 15 shows that the non-dormant em- 

 bryo gives a much bigger plant after 14 days' growth than the embryo 

 from the dormant seed gives after 32 days' growth. Likewise, the non- 

 dormant embryo gives much more growth after 21 days of growth than 

 the embryo from the dormant seed gives after 47 days. As we shall see 

 later, so-called dormant embryos are not generally incapable of growth if 

 the coats are removed, but they have so little vigor of growth that they 

 cannot overcome the slight resistance of even so thin a coat as the cockle- 

 bur seed bears. In general, also, dormant embryos, if forced to grow, pro- 

 duce dwarfish plants so far as the above-ground portion of the plant is 

 concerned. The root system is not dwarfish but is large in comparison 

 with the top. In the next chapter we shall have much more to say about 

 the effect of oxygen pressure on germination and dormancy induction, 

 about the importance of seed and fruit coats in dormancy of seeds, and 

 about dormant embryos. 



Davis was able to throw seeds (embryos) into and out of dormancy 

 repeatedly and at will. Nature induces dormancy in embryos of some 

 species of plants when they mature. Many seeds after-ripen in nature in 

 the soil during the cool weather of fall, winter, and spring. If Davis and 

 nature use the same methods in inducing and overcoming embryo dor- 

 mancy, embryos in nature are thrown into dormancy as the seeds mature 

 because of the relatively high temperature during maturing and because 

 of limited oxygen supply to the embryo caused by seed coats and other 

 tissues about embryos. As we shall see in the next chapter, many seeds 

 in the temperate zone after-ripen in soil at temperatures slightly above 

 freezing. Do many seeds in the soil after-ripen in nature in the cool weather 

 of fall, winter, and spring, which gives an abundance of germination in 

 early spring? Does the hot weather of summer throw many of the seeds 

 in the soil back into the dormant condition to be after-ripened again the 



