ELEMENTS OF AGRICULTURE. 
to 
cellular integument . 3. A vascular layer . 4. A whi¬ 
tish layer, apparently of a fibrous texture, the inner bark, 
which is of a more complicated structure than the other 
layers. Then the imperfectly formed wood, called albuv 
num ; the wood itself; and finally the pith. 
32. The leaves are those parts of the plant attached to 
the stem, or branches, and from which they generally be¬ 
come detached every year. Some are united to the vege¬ 
table by means of a stem, called footstalk ; and others are 
attached immediately to the plant, without any interve¬ 
ning link. Many naturalists suppose that it is through the 
under sides of their leaves that plants absorb the gases 
that aid in their nutrition. 
33. Vegetables procure the elements of their subsistence 
not only through their roots, but also through their stems 
and leaves, which in this case play a very important part. 
They absorb, through their pores,* the nutritious gases 
contained in the atmosphere. The quantity, more or less, 
of these principles absorbed, depends upon the organization 
of the plant, the manner of its cultivation, and the state 
of the atmosphere. 
34. It is principally hydrogen and azote, combined in 
the shape of ammonia and carbonic acid,f that is absorbed 
« 
* The minute particles that compose a body are called molecules, and the 
interstices, or open spaces between these molecules, are called pores. AL 
bodies have pores, and are on that account called porous. 
t Air is composed of two principal bodies, oxygen and azote. The result 
of a combination of oxygen with a simple body, is called an acid. Acids are 
easily recognised by a sour taste, and the property which they possess of 
turning vegetable blues to red. Their names differ as the quantity of oxy¬ 
gen entering into their composition is greater or less. Thus, when it exists 
in the greatest proportion possible, we terminate the name of the simple 
body in ic : but if the quantity of oxygen is not so great, the name of the 
simple body is terminated in ous — e. g., sulphuric acid , sulphurous acid: nit * 
vie acid , nitrous acid. When we bum a match, we obtain sulphurous acid % 
and sulphuric acid is the liquid substance usually called oil of vitriol. If the 
body formed by the combination of oxygen with another simple body does 
not possess the properties indicated above, it is called an oxyde. Amcng the? 
2 * 
