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 fi:-om 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 nature for 
that purpose, as in Canna, Commelina, &c. If the radicle 
