132 GROWTH OF PLANTS. 



at or below the freezing point, and hence seeds will 

 not germinate in such circumstances. When the tem- 

 perature, again, is too high, that is, above 90 Fahr. 

 the parts and fluids become too much expanded ; and 

 hence it has been found by experiment, that from 132 

 to 144 is unfavourable to germination. 



Air is likewise indispensable to germination, though 

 Homberg alleges that he made some seeds germinate 

 in the exhausted receiver of an air-pump, an experi- 

 ment, however, not since verified, and Saussure deemed 

 it inaccurate. It is on this account that seeds long 

 buried in the earth lie dormant, but when turned up 

 by digging or otherwise, soon germinate. Oxygen is 

 the most indispensable requisite, for seeds placed in 

 azote, or in carbonic acid gas, will not germinate. In 

 pure oxygen gas, and as Humboldt found in a solution 

 of chlorine, germination is very rapid, but the young 

 plants thus produced soon perish. 



Light, from its causing the disengagement of oxy- 

 gen, and the retention of carbonic acid gas, is unfa- 

 vourable to germination, because there is wanted the 

 fixing of the oxygen, and the disengaging of carbonic 

 acid gas. When the gemlet or plumule once gets to 

 a certain size, however, it counteracts, according to 

 Mirbel, the fermentation which light may have caused 

 to begin, by converting the starch of the embryo into 

 sugar, and an emulsive fluid. 



The time required for germination varies much in 

 different species : thus, mustard takes li ttle more than 

 one day ; cress, two days ; spinach, turnip, and kidney 

 beans, three days; lettuce, four days; most grasses, 



