PHYSIOLOGY. 281- 
plan in vegetables, which, as we know, take in food, 
and exhale gaseous fluids? Were we quite st-an- 
gers to the structure of the organs and vessels in the 
vegetable kingdom, we might however be able to 
draw that conclusion a priori. But we know their 
structure, and need not form hypotheses, as we are 
acquainted with the nature of the air-vessels. They 
act, at the same time, asthe trachea and as the in- 
testines of plants. ‘The radicles or fibres of the 
roots, consist almost entirely of air-vessels. “They 
imbibe, with their spirally winding channels, the 
necessary moisture. The hollow air-vessels carry 
carbonic acid gas, which has become free, through 
caloric as well as oxygen gas. ‘They convey the 
whole to the root. The vital power fixes the car- 
bon, and decomposes the water, (§ 278). 
Vhe chief food of plants consists of carbon and 
hydrogen. ‘The hollow air-vessels carry the oxygen 
gas, which was formed during the day, out of the 
plant, and at night time, when the rays of the sun 
are wanting to evolve more oxygen gas, they exhale, 
through the pores of the cutis, carbonic acid gas, 
which they received from the ground, and which, 
for want of light, they could not keep fixed. The 
more convoluted vessels, by means of those con- 
volutions, prepare, by aid of light, the secreted 
juices, and carry the rest, in form of thin vapours, 
off through the pores of the cutis. These apertures 
or pores, which have valves, by which they may close 
and shut themselves, are certainly the ends of the 
air-vessels ; at least we may suppose this with cer- 
gainty almost, though ocular demonstration is still 
4 | wanting. 
