"How lucid!" said Amy, with laughing irony. "Then," she added, "please
take nothing for granted except my ignorance in these matters. I don't
know anything about plants except in the most general way."

"Give me time, and I think I can make some things clear. A plant breathes
as truly as you do, only unlike yourself it has indefinite thousands of
mouths. There is one leaf on which there are over one hundred and fifty
thousand. They are called _stomata_, or breathing-pores, and are on
both sides of the leaf in most plants, but usually are in far greater
abundance on the lower side. The plant draws its food from the air and
soil--from the latter in liquid form--and this substance must be
concentrated and assimilated. These little pores introduce the vital
atmosphere through the air-passages of the plant, which correspond in a
certain sense to the throat and lungs of an animal. You would be sadly
off if you couldn't breathe; these plants would fare no better. Therefore
we must do artificially what the rain does out-of-doors--wash away the
accumulated dust, so that respiration may be unimpeded. Moreover, these
little pores, which are shaped like the semi-elliptical springs of a
carriage, are self-acting valves. A plant exhales a great deal of
moisture in invisible vapor. A sunflower has been known to give off three
pounds of water in twenty-four hours. This does no harm, unless the
moisture escapes faster than it rises from the roots, in which case the
plant wilts, and may even die. In such emergencies these little stomata,
or mouths, shut up partly or completely, and so do much to check the
exhalation. When moisture is given to the roots, these mouths open again,
and if our eyes were fine enough we should see the vapor passing out."

"I never appreciated the fact before that plants are so thoroughly
alive."