Dormancy and Germination of Seeds 267 



seed coats or fruit coats inclosing these seeds are either imperme- 

 able to oxygen or at least retard the entrance of oxygen to such 

 an extent that germination cannot take place. The best-known 

 example of this class of seeds is the cocklebur, a weed very diffi- 

 cult to eradicate in low grounds. The dry fruit of the cocklebur 

 contains two seeds, an upper and a lower. In nature the lower 

 seed usually germinates in the spring following maturity, while 

 the upper seed does not grow until later, or not until the second 

 spring. Both seeds require considerable oxygen to germinate, 

 but the upper requires about one third more than the lower. So 

 long as the seed coats remain intact, no germination is possible ; 

 but if the embryos are removed, they germinate at once. In 

 nature, freezing and thawing and the action of the bacteria and 

 fungi of the soil alter the seed, so that after a few months suffi- 

 cient oxygen penetrates the lower seed to start growth in the 

 embryo ; but it requires more than a year for the seed coats of 

 the upper seed to decay, or to become altered sufficiently for 

 enough oxygen to reach the embryo to induce growth. Many 

 grasses — for example, the Johnson grass — and many com- 

 posites have seeds whose behavior is similar to that of the cockle- 

 bur. Seeds of this kind have been found to germinate rather 

 readily when kept for a time at a temperature above 120° F. 



4. Embryos requiring acidity. The seeds of the peach, haw- 

 thorn, red cedar, hard maple, basswood, ragweed, and to a less 

 extent apple seeds, will not germinate when first matured, even 

 if the seed coats are removed. In these seeds dormancy is due 

 to the condition of the embryo. Experiments have shown that 

 the stem-part (hypocotyl) of the embryo is neutral or alkaline in 

 reaction in the mature seed, and that dormancy lasts as long as 

 this condition lasts. As soon as the whole embryo becomes acid 

 in reaction, the seed will germinate. 



Seeds of this class may be germinated at once by removing 

 the seed coat and immersing the embryos in weak acids. It has 

 long been the practice of horticulturists to '' layer " such seeds; 



