302 PHYSIOLOGY. BOOK II, 



for a quarter of an hour. But in dry air many seeds bore the 

 temperature of 75 "^ Cent. (167° Fahr.) for a quarter of an 

 hour without inconvenience. Hence it appears that seeds in 

 steam can bear 12° Cent, more than in water, and in dry air 

 13° Cent, more than in steam. 



In these experiments the action of temperature was ex- 

 tremely rapid. In lowering the temperature and prolong- 

 ing its action, it was found that when Wheat, Rye, and 

 Barley were exposed for three days on water to a tempera- 

 ture of 35° Cent. (95° Fahr.), four-fifths of the Wheat 

 and Rye, and all the Barley, were killed. Hence it would 

 appear that 35° Cent, form the highest limit of temperature 

 which corn can bear under such circumstances. But in sand 

 or earth the same grains sustained a prolonged temperature of 

 40° Cent. (104° Fahr.) without inconvenience; at 45° Cent. 

 (113° Fahr.) a great part perished ; at 50° Cent. (122° Fahr.) 

 the whole of them. 



These remarkable experiments are calculated to throw 

 great light upon the cause of the impossibility of making- 

 certain plants multiply themselves by seeds in hot countries. 

 If Wheat, Barley, &c., cannot endure a prolonged tempera- 

 ture above 40° Cent., and the temperature of the soil is in 

 some countries and soils as high as 60° Cent. (140° Fahr.), as 

 Humboldt asserts, or between 48° and 53° Cent. (122° Fahr.), 

 even in some parts of France, as Arago states, — it is evident 

 that the seeds of corn placed in such situations will perish. 



Exposed to the influence of water, heat and air, the parts 

 of a seed soften and distend ; the embryo swells and bursts its 

 envelopes, extending the neck and the bases of the cotyledons, 

 and finally emitting its radicle, which pierces the earth, de- 

 riving its support at first from the cotyledons or albumen, but 

 subsequently absorbing nutriment from the soil, and commu- 

 nicating it upwards to the young plant. The manner in which 

 the embryo clears itself from its integuments differs in various 

 species : sometimes it dilates equally in all directions, and 

 bursts through its coat, which thus becomes ruptured in every 

 direction ; more frequently the radicle passes out at the hilum, 

 or near it, or at a point apparently provided by naturae for 

 that purpose, as in Canna, Commelina, &:c. If the radicle 



