CH. I.] SOWING. 13 



This brings me to the consideration of the con- 

 tingencies necessaiy to cause a seed's germination. 



A certain degi'ee of warmth is essential, for no 

 known plant has seeds that will genninate below or 

 at the freezing point of water. A temperature above 

 32° of Fahrenheit's thermometer, therefore, is 

 requisite ; and the plants whose seeds will germinate 

 nearest to that low degree of temperature, in this 

 country, are the winter weeds. For example, I have 

 found the seeds of the Poa annua, the commonest 

 grass of our gravel walks, germinate at SS"^, and the 

 seeds of groundsel (Senecio vulgaris) would probably 

 requii'e no higher temperature. But, on the other 

 hand, the temperature must not be excessively high. 

 Even no tropical seed, probably, will germinate at a 

 temperature much above 120^ F., and we know from ' tl^^f.i ix^ 

 the experiments of MM. Edwards and Colin, that ' ^ 



neither wheat, oats, nor barley will vegetate in a 

 temperature of 113°.^ 



Every seed differing in its degree of excitability, 

 consequently has a temperature without which it 

 will not vegetate, and from which cause arise the 

 consequences that different plants require to be so"uti 

 at different seasons, and that they genninate with 

 various degrees of rapidity. 



For example, two vaiieties of early pea, sown on a 

 south border on the same day, and treated strictly 



^ Jour, de Pharmacie, xxii. 210. 



