Functions of Vegetables* 199 ? 
cessary to this- process. No vegetation whatever takes 
place when the temperature is at the freezing point, and 
very little till it rises many degrees above it. Air is 
no less requisite for the germination of seeds ; when it 
is entirely excluded, as in the vacuum of an air pump, 
no change takes place. Moisture is also necessary in 
this process ; hut in most cases water must be applied 
in a regulated and moderate quantity, — for, excepting 
the seeds of aquatic plants, which possess peculiar ha« 
bits, most seeds, when exposed to excessive moisture, 
are deprived of their vegetative power. The exclusion 
of light is favourable to the vegetation of seeds, and 
hence it is that their germination is greatly promoted 
by covering them with the soil. 
When a seed begins to germinate, the first change 
observed is the increase of size by the absorption of 
moisture ; the radicle, or little root, pushes out and 
stretches downwards into the earth, from which it con- 
veys nourishment for the growth of the future plant. 
Another part, called plumula, shoots upwards, and fi- 
nally expands into leaves and branches; but these_ re- 
markable effects are owing to certain changes which 
take place within the seed. The absorption of the oxy- 
gen of the atmosphere, the evolution of carbonic acid 
gas, by the combination of the oxygen with the car- 
bone of the seed, and the conversion of the farinaceous 
matter into a saccharine substance, which is destined 
for the nourishment of the embryo plant, are the first 
changes observed in the germination of seeds. The 
cotyledons, or seed-lobes, are to be regarded as store- 
houses of food for the young plant, before the evolu- 
tion of its parts are fully completed, to enable it to de- 
rive nourishment from the earth. 
