CH. I.] 



SOWING. 



45 



The necessaries in search of which their upward 

 course is directed, are air and hght, but especially 

 the first, for the plantlet rises above the surface 

 though the seed is genninated in a totally dark room, 

 but if the seed of an aquatic plant be germinated in 

 water under a double glass receiver, one compartment 

 of wliich is filled with hydi'ogen or nitrogen, and the 

 other compartment with atmospheric air, the plantlet 

 invariably dii'ects its growth into the latter. We also 



know that germinated seeds placed in vacuo refuse 

 to advance any further in vegetation. 



The absorption of moisture and the consequent en- 

 largement of the cotyledons is followed by another 

 change in them. Oxygen gas is absorbed and carbonic 

 acid is evolved, the farinaceous natiu'e of the seed being 

 completely changed — it usually becoming saccharine, 

 though sometimes it attains acidity — but in every 

 case its components become soluble in water, more 

 liquid, and adapted to the nutriment of the embryo 



