I.] VITALITY OF SEEDS 13 



process necessary to maintain life exhausts all the 

 available material in the embryo, which then perishes, 

 but long before death the vigour of the seed becomes 

 greatly weakened by age. There seems some evidence 

 that seeds buried in the ground may retain their vitality 

 for much longer periods than seeds stored in the open 

 air, perhaps because the greater supply of moisture 

 enables the embryo to draw upon the food store to 

 support the combustion process, instead of dying as soon 

 as it has used up all its own stock ; but the evidence that 

 long-buried seeds do retain their vitality is still a little 

 doubtful. Of course, germination of the so-called 

 " mummy wheat " is a fairy tale ; whenever wheat grains 

 are found that were indubitably buried with the mummy, 

 not only has the embryo perished but the material of 

 the endosperm is carbonised just as if it had been 

 charred, so that it is no longer capable of forming food 

 for an embryo. 



Since the food store of the cotyledons or endosperm 

 exists for the purpose of maintaining the embryo until it 

 has built up a plant capable of carrying on an independ- 

 ent existence, it is clear that if this latter process is too 

 long deferred, the food store may give out and the plant 

 perish. The larger the seed the more reserve there is, 

 and the longer the young plant can grow on the seed 

 store alone ; for this reason the depth at which seeds 

 can be sown is determined by their size. Provided it 

 does not get so far down as to be cut off from the daily 

 warmth of the sun or from the air, the deeper a seed is 

 sown the better, because it is thereby more sure of 

 obtaining a continuous supply of moisture. But minute 

 seeds, like those of a poppy, do not possess enough 

 material to give rise to more than a very short stem or 

 leaf with which to reach up to the light : mustard seeds 

 can hardly struggle up 3 inches, while 6 inches taxes 



