68 GROWTH OF PLANTS 



to induce dormancy in embryos. Later we shall discuss some of the physi- 

 cal and chemical changes occurring in embryos during after-ripening and 

 during dormancy induction. As we shall see later, many other changes 

 involved in overcoming dormancy also take much time and involve kno^vn 

 biochemical changes and, like those mentioned above, can be interpreted 

 on the basis of definite physical and chemical changes rather than by the 

 vague term "stimulus." 



Later in this chapter under the topic "Categories of Dormancy" we 

 shall point out many ways in which nature maintains dormancy in a 

 moist soil, and several means by which seeds are throAvn out of dormancy. 

 Contrasted mth the old explanations of seed dormancy caused by hard- 

 coatedness on one hand and lack of a release stimulus on the other, the 

 new data and explanations afford a much greater richness of concept and 

 precision of conclusions. 



Advantages to Plants of Delayed Germination 



Delayed germination is advantageous to many wild plants in nature. 

 It carries the plant over the winter in the seed stage and the young plant 

 grows in the spring. In the Dakotas, Minnesota, Saskatchewan, and ad- 

 joining regions wild oats are a bad weed because the grains are suffi- 

 ciently dormant to carry over intact until spring, when they germinate. 

 Cultivated oats and false ^vild oats are not troublesome weeds in these 

 regions because the dormancy of these grains is so temporary that the 

 grains after-ripen and germinate in the fall and the cold winter kills the 

 seedlings. In the previous chapter we learned that seeds of some \vild 

 plants lie dormant in the soil for years, or even centuries, and germinate 

 only when the soil is stirred up or burned over. This is no doubt helpful 

 in the persistence of the species. 



Advantage to Man of Dormancy in Seeds 



It would be a calamity for mankind if all at once seeds of cultivated 

 plants ceased to have at least a temporary dormant period. Mangels- 

 (JQpf 79, 80 found a number of types of maize, produced either by inbreed- 

 ing or by crossing, in which the grains had no dormant period, but instead 

 the embryos continued to grow in the green ear and to form seedlings. 

 Professor William H. Eyster ^^ has kindly furnished an illustration of 

 such an ear of corn which is shoAvn in Fig. 2L Pope and Brown ^^ forced 

 young embryos in heads of normally very dormant varieties of barley to 

 continue to enlarge and form seedlings in the green head by placing moist 

 filter paper on the embryo portion of the immature grain. Apparently 

 water deficit in the green head is a factor in inducing dormancy. Fig. 22 

 shows viviparous heads of barley, the photograph for which was very 

 kindly supplied by the authors. Evidently appearance of dormancy in 

 grains of cereals, or its failure to appear, is determined either by genetic 

 characters, as shown by Mangelsdorf and by Eyster, or by conditions of 



