18 
green color of vegetation is due to a peculiar green matter, lying loose in the 
cells, in the form of minute grains, named chlorophyl, which is merely the Greek 
for the green of leaves. The use of the leaves is to convert the crude watery 
matter sucked up by the roots into proper vegetable substance. They have been 
compared, because of this, to the digestive organs of animals, and, as their func- 
tions are respiratory as well, they have been compared also to the lungs. 
A large portion of the moisture which the roots of a growing plant are 
constantly absorbing, after be‘ng carried up by the stem, is evaporated by the 
leaves. ‘To prevent a too copious evaporation of this moisture, they have been 
provided with the delicate tissue of cells termed the epidermis. 
The greater part of the moisture exhaled escapes from the leaf through the 
stomata or breathing pores ; these are small openings through the epidermis into 
the air chambers, establishing a direct communication between the air chambers 
and the external air; through these, the vapour of water and the gaseous con- 
stituents of the atmosphere can freely enter or escape, as the case may be. The 
openings of these breathing pores are guarded by a pair of thin walled cells, 
which open when the weather is damp, so as to allow exhalation to go on; but 
promptly close when it is dry, so as to arrest it before the interior of the leaf is 
injured by the dryness. ‘These stomata or mouths, too, are the entrances 
through which the plant receives its food. I have already mentioned that the 
chief materials which compose the food of a plant are carbonic acid, water—that 
is hydrogen and oxygen—and a little ammonia. ‘The substance of which vege- 
table tissue, namely the walls of the cells, is made, is, by chemists, called cellulose, 
and is just the same in composition in the hardest wood of a full grown tree as 
in the soft cellular tissue of a succulent plant; it is composed cf carbon, hydro- 
gen and oxogen. These, then, are necessary materials for the vegetable growth, 
and must be received by all growing plants, and it only remains to be shewn 
whence the plant obtains them. Of all substances consumed by a plant, water 
is taken in to a far greater extent than any other, chiefly through the roots, but 
also, to asmall degree, through theleaves, Now, wateris composed of oxygen and 
hydrogen, two of the three elements of cellulose, and, strangely enough, these 
two gases exist in water in exactly the same proportion as they do in cellulose. 
It is perfectly apparent, then, that the plant obtains two of the three elements of 
cellulose from water, the other, carbon—by far the most important—is the only 
one now to be found. 
Among the components of the atmosphere, there is one which is exceedingly 
injurious to animal life, which is called Carbonic aeid, or more usually now 
Carbonic dioxide; this gas consists of carbon and oxygen and is the product of 
combustion. In breathing, which is one form of combustion, animals are 
constantly forming carbonic acid gas by uniting the carbon of their bodies with 
the oxygen which they inspire ; it is then exhaled as carbonic acid gas. It. 
generally exists in the atmosphere to the amount of ,,1,, part of its bulk. This 
is not enough to prove injurious to animal life. But, with every breath, animals 
