i5o6 A TEXTBOOK OF THEORETICAL BOTANY 



others were still resistant and seedlings kept appearing at intervals during 

 fourteen subsequent years. Seeds of the Poppy have a remarkably long 

 dormancy and therefore accumulate in the soil year after year. Even after 

 two years, in the above experiment, only about half of the Poppies had 

 germinated. Grass seeds, on the other hand, have mostly only a short 

 period of obligate dormancy. 



Periodicity in germination is characteristic of many weeds, the majority 

 germinating in autumn, after the summer heat (winter annuals), and a 

 minority in the spring (summer annuals). Even in a heated greenhouse 

 periodicity is maintained although, in this case, there is a larger germination 

 in the spring months than under exposure to seasonal changes of tem- 

 perature. 



Dormancy must be distinguished from longevity, which means the 

 period during which seeds can survive induced dormancy, under certain 

 conditions, and remain viable, that is to say capable of germination. Ewart 

 distinguished three main classes of seeds in this respect: microbiotic, those 

 with a longevity not exceeding three years; mesobiotic, those which survive 

 more than three but not more than fifteen years; and tnacrobiotic, those which 

 survive from fifteen to a hundred years or more. Such a classification 

 requires, however, a knowledge of the optimum conditions for survival, 

 which we only have in a few cases. 



There are many stories of seeds apparently surviving for fabulous 

 periods. The most famous of these, the legendary mummy wheat from 

 ancient Egyptian tombs, is revived from time to time but has never stood 

 the test of investigation. True mummy wheat looks sound externally, 

 but the embryo has gone and only the starchy endosperm remains. The 

 oldest viable seeds known with certainty are those of the Lotus, Nelumbo 

 micifera, which were found by Ohga embedded in peat on the site of an 

 ancient lake in Manchuria. When the coats were filed through, they gave 

 ICO per cent, germination. Measurements of the radio-active carbon content 

 have shown these seeds to be between 8co and 1,250 years old. They have 

 exceedingly hard, impervious coats, which must be broken or decayed 

 before the seed can germinate. Most of the seeds found had partly decayed 

 coats and probably all would eventually germinate if not too deeply buried. 



We have already drawn attention to the very long survival of many 

 seeds of weeds in soil, a fact which is familiar to gardeners and excavators. 

 The Poppies which blazed across the shell-toin land of Flanders in the 

 First World War have been immortalized in remembrance, but the Snow- 

 drops which covered the trenches around Sebastopol in the Crimean War 

 were an earlier instance which has been forgotten. Tiny seeds like these 

 soon wash down deep into the soil, where they may rest for unknown periods 

 until some disturbance restores them to the air. 



Storage in soil is often more favourable to survival than dry storage, 

 but dry seeds can also survive for remarkable periods. Seeds of Nelumbo 

 had been in store in the British Museum for 150 years and hard-coated 

 seeds of Albiszia julibrissin for 149 years in the herbarium, when they 



