THE ANGIOSPERMAE 1505 



ages. Some indeed have no dormancy at all and will sprout in damp 

 conditions even before they have been shed. Even in these highly selected 

 strains of seed, however, variations occur with hard coats, which may only 

 germinate after considerable delay. 



Many seeds will undergo the period of after-ripening successfully in the 

 drv state, but there are others which can do so in the moist state and a 

 considerable number must be held in the moist state as they cannot toler- 

 ate drying. In all these cases the term dormancy properly applies to 

 that period, which may be short or long, during which the seed is incapable 

 of germination under natural conditions, and ends when the seed becomes 

 germinable, which, again under natural conditions, usually means that 

 germination takes place. Thus true or obligate dormancy should be dis- 

 tinguished from induced dormancy due to the absence of the necessary 

 conditions for germination. 



The seeds of wild plants are very variable in their periods of dormancy. 

 Even seeds from the same ovary may behave very differently and their 

 germination times be spread over a period of several years. This variability 

 is chiefly advantageous in uncertain climates and it is noteworthy that 

 tropical seeds usually have short dormancies. When one considers that in 

 general the seeds of wild plants fall into the soil, it is perhaps surprising 

 that so many seeds will ripen in dry storage, but it must be remembered 

 that moisture does not necessarily enter the seed. Seeds capable of pro- 

 longed dormancy in the soil usually have impermeable coats and do not 

 absorb water or become germinable until the testa has either been mech- 

 anically injured or else has sufficiently rotted to become permeable. Even 

 where this is not the case the seed may have special requirements, such as a 

 period of low temperature or exposure to light, which are needed to render it 

 germinable. The need for a period of exposure to low temperature is often 

 the reason why some seeds, called " two-year seeds", will not germinate 

 until the second spring after their maturation. Exposure to low temperature 

 is sometimes also needed to overcome epicotyl dormancy, which occurs in 

 some plants after germination has begun and a root has started to grow. 

 No shoot is produced until the seedling has been chilled for two or three 

 months. This curious condition has been observed especially in seeds of 

 Lih'ufn, Paeonia and Viburnum. 



Both high and low temperatures may sometimes be required, as in 

 Cotoneaster, the former to promote decay of the testa and the latter to allow 

 after-ripening. 



Dormant seeds are present in most soils to a surprising extent and may 

 remain viable for very long periods. At Rothamsted Experimental Station 

 there is a field known as Broadbalk which has grown wheat every year 

 since 1843. In 1925 a system of sectional fallowing was adopted to reduce 

 the weeds. Samples of the soil were placed under greenhouse conditions 

 and carefully observed for seed germination. The weed-seed population in 

 1925 was actually 300 million per acre. Most of the seeds present were 

 germinable and their seedlings appeared during the first three months, but 



