392 PLANT PHYSIOLOGY 



similarly and germinate readily once the testas are removed and 

 oxygen is allowed to enter freely. 



On the other hand, some seeds like those of Typha (cat-tail) 

 germinate better under water than in air, and only when the 

 oxygen pressure is reduced do they germinate well. These seeds 

 showed 96% germination in four days when 40-80% of the air 

 was replaced by hydrogen or nitrogen, while in ordinary air no 

 seeds germinated in this period and only 4% after ten days. This 

 favorable effect of reduced pressure disappears when the testas 

 are broken. This characteristic has evidently had survival value 

 for seeds which ordinarily germinate in water and in swamps, 

 where the oxygen pressure is abnormally low. 



4. The embryos are not mature. Such seeds have very small 

 embryos when the seed is developed, after which the slow process 

 of maturation may continue, in some cases, for many weeks. 

 Seeds of this class occur over a wide range of families and include 

 the Ginkgo, Ilex (holly), Ranunculus ficaria (buttercup), Erythro- 

 nium (dogtooth violet), Corydalis, and many orchids. 



5. The embryos require a period of " after ripening.' ' Such 

 seeds will not germinate even though the embryo has attained 

 its full size and all the other obstacles mentioned above have 

 been removed. In the apple (Pyrus), peach (Prunus), hawthorn 

 (Crataegus), maple (Acer), basswood (Tilia), bitter-sweet (Celas- 

 trus), red cedar (Juniperus) , and ragweed (Ambrosia), the embryo 

 is neutral or alkaline in the mature seed, and dormancy persists 

 until the reaction of the embryo is acid. The acidity is prob- 

 ably necessary to favor the activity of the enzymes concerned 

 in germination, which may hence be hastened by treating the 

 seeds with dilute acids. A low temperature (5° C.) is also favor- 

 able to the development of acidity. In nature these changes 

 are brought about by freezing and thawing and contact with 

 the soil. 



6. Secondary dormancy has set in. After seeds have become 

 ripe and are able to germinate, it may happen that this capacity 

 is lost and the seeds must be ripened again. This is secondary 

 dormancy, and has been observed in Brassica alba and Ambrosia 

 trifida. This type of dormancy is related to the oxygen supply 

 and seems to be caused by the fact that the impermeability of 

 the surrounding membranes to the carbon dioxide produced in- 

 side and to the oxygen outside causes a reversal or a loss of the 



