, BOTANY OF THE LIVING PLANT 



Latent Period. 



After seeds are shed and distributed they usually undergo a period 

 of rest. During the autumn and winter of temperate climates they 

 ome buried in the soil. They naturally fall into chinks and 

 crannies, and are often covered by rotting leaves ; they are also washed 

 into the soil by rain, or drawn below by the restless activity of earth- 

 worms, or covered by their castings. But some work their own way 

 into the soil by hygroscopic movements, as in Avena, or Stipa, or 

 Erodium. A few even bury their fruits as they mature by geotropic 

 curvature of the fruiting stalk, as in the Earth-nut (Arachis), or the 

 subterranean Clover. In one way or another they become covered, 

 and in the dormant season there is plenty of time for the process. 



A latent or resting period is commonly determined by climatic 



conditions of drought in the tropics, or of cold in temperate climates. 



But a latent period may be self-induced, in which case the seeds will 



not germinate till after a period of rest. In this respect seeds vary. 



Some will germinate immediately they are matured if the conditions 



are favourable. It is this that makes the difficulty with cereal crops 



in a warm, wet autumn : for those conditions stimulate immediate 



germination, and the grain is liable to sprout in the stook before it 



can be harvested. But as a rule a period of rest follows on ripening. 



The seed dries out, and in that state it remains stationary, but retains 



its vitality, being specially resistant to extremes of drought and 



heat. Examples of an obligatory resting period have been quoted 



in Chapter IX., p. 140. If such seeds are collected in autumn, and 



exposed to conditions favourable for germination, they remain passive 



till the spring, when they will germinate almost simultaneously. 



Other plants again have, in addition to seeds that germinate in the 



first year, other seeds which require a longer rest : Laburnum, wild 



Mignonette, and field Clover are stated to be among these, while 



the Cockleburr (Xanthium), with its two fused fruits, is also said to 



germinate one in the first, the other in a later season. 



4 



Akin to such questions is that of the length of time during which seeds can 

 retain their vitality. This varies in the individuals of any sample of seeds 

 saved under apparently uniform circumstances. As the period is lengthened 

 the proportion of seeds that germinate diminishes. Oily seeds retain their 

 vitality a shorter time than starchy seeds ; and those kept dry retain it 

 longer than those kept damp. But to extend this possibility of survival 

 to the so-called "mummy-wheat" is too long a step to seem probable. A. 

 de Candolle, after examining the evidence up to 1882, concluded that no 



