CH. I.] SOWING. 29 



his house-plants out until the mulberry leaf was of a 

 certain growth." 



As no seed ^^ill germinate unless a certain degree 

 of heat is present, so also does it require that a cer- 

 tain quantity of water is in contact ^^ith its outer 

 skin or integument, and this is required not only to 

 soften this covering, and thus permit the enlarge- 

 ment of the cotyledons (seed lobes) always preceding 

 germination, but also to afford that water to the in- 

 ternal components of the seed, without which the 

 chemical changes necessary for the nutriment of the 

 embryo plant will not take place. 



Pure water, or some other liquid of which it is 

 a large constituent, is absolutely necessary ; no other 

 fluid will advance germination a single stage. The 

 quantity of water, necessaiy to be present before 

 germination will proceed, varies much. The seeds 

 of aquatic plants require to be completely and con- 

 stantly submerged in water; others, natives of dry 

 soils and warm climates, \vill germinate if merely 

 exposed to a damp atmosphere, of which the Spanish 

 and Horse chestnut afford ready examples ; but the far 

 larger majority of seeds require and germinate most 

 healthily in contact with that degree of moisture 

 wliich a fertile soil retains only by its chemical and 

 capillary attraction. If the soil be inefficiently 

 drained, and there be, consequently, a superfluity of 



