CHAP. X. 
SEED. 
363 
corn can bear under such circumstances. But, in sand or 
earth, the same grains sustained a prolonged temperature of 
40°centig. (104°Fahr.) without inconvenience; at 45°centig. 
(113°Fahr.) a great part perished; at 50°centig. (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, See., cannot endure a prolonged tempera- 
ture above 40° centig. ; and the temperature of the soil is in 
some countries and soils as high as 60° centig. (140° Fahr.), 
as Humboldt asserts, or between 48° and 53° centig. (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 nature for 
that purpose, as in Canna, Commelina, &c. If the radicle 
has a coleorhiza or root-sheath, this is soon perforated by 
the radicle contained within it, which passes through the 
extremity ; as in Grasses, and most monocotyledonous plants. 
The cotyledons either remain under ground, sending up their 
plumule from the centre, as in the Oak ; or from the side of 
their elongated neck, as in Monocotyledons; or they rise 
above the ground, acquire a green colour, and perform the 
ordinary functions of leaves, as in the Radish and most plants. 
In the Mangrove, germination takes place in the pericarp, 
before the seed falls from the tree ; a long thread-like caulicle 
is emitted, which elongates till it reaches the soft mud in which 
