422 THE BIOLOGY OF FLOWERING PLANTS 



retain vitality may be taken as an expression of its powers 

 to resist extreme conditions. Perfectly dry seeds can, in 

 fact, withstand not only the implied drought, but also 

 remarkable extremes of temperature. The temperature 

 of Hquid hydrogen ( —234° C) has no harmful effect, and 

 many seeds can withstand a temperature of 100° C. for two 

 days, though the water-saturated seed is quickly damaged 

 by much lower temperatures. In nature it may be necessarj'^ 

 to withstand desiccation for long periods, especially in arid 

 climates, where, too, the temperature of the surface soil 

 may rise very high. In temperate countries, however, the 

 seed is probably not thoroughly dried out after it has reached 

 the soil ; it may endure low, though not very low, temper- 

 atures. The maturation of the seed and its separation from 

 the parent plant entail a cessation of the water supply, and 

 it is in relation to this that the seed has evolved as a dry 

 structure with limited reactivity ; from this has arisen its 

 great and prolonged power of resistance as a secondary 

 function which has come to be of great importance. 



The forced dormancy of seeds in the soil or under water 

 has a somewhat different significance. While seeds, like 

 those of many leguminous plants, with impermeable seed 

 coats really present cases of inhibition due to desiccation, 

 the others may be regarded as being held dormant in 

 conditions which would be unfavourable to the develop- 

 ment of the young plant ; the seed immersed in water, or 

 buried deeply in the soil is a case in point. Perhaps it is 

 of even greater importance that the whole of the crop does 

 not germinate at once. The longevity of the seed, combined 

 with dormancy, means that the offspring of a single parent, 

 or year, may go on germinating through a series of seasons ; 

 and this may be a real advantage. 



§ 2. Dormancy of Seeds 



Apart from forced dormancy, as through carbon dioxide 

 narcosis, many seeds, from the nature of the embryo or the 

 structure of the coats, require a more or less prolonged 



