114 THE VEGETABLE WORLD. 
The sources of ammonia are nearly the same as carbonic acid : 
the secretions of animals, the combustion of wood and coal, and 
the decomposition of nitrogenous matter. 
The constituents of sap in special plants are the metallic oxides, 
as potassium, sodium, calcium, and magnesium. How these various 
substances permeate through the tissues of plants, is, to a certain 
extent, a mystery. : 
On THE GrRowTH OF VEGETABLES. 
Vegetables increase by means of the nourishing materials elabo- 
rated by the descending sap. It would be entering upon a long 
and difficult task to take into consideration all the points which 
botanists have made concerning the growth of vegetables. We 
must, therefore, limit ourselves here to the consideration of 
what actually comes under our own knowledge in the growth of 
trees. Trees are elongated by the development of their terminal 
shoots. We observe the intervals between the points of the 
insertion of the leaf, which at first are very short, gradually 
become longer according to the greater degree of development 
through which they have passed. 
But how do trees increase in circumference ? This demands @ 
closer examination. 
If we study the internal structure of a branch of one year old 
in one of our forest trees, we shall find that towards the end of the 
year the branch is arranged in the following manner. There is @ 
pith, a fibro-vascular circle, furnished with air-vessels on its 
internal face, with straight medullary rays traversing the ligneous 
_ circle, losing themselves in the bark, which is composed of an 
epidermis of ruber, of the herbaceous envelope, and of liber. But 
between the ligneous and cortical system we can distinguish the 
presence of a special zone formed of very delicate cells, with walls 
soft and transparent, which in spring-time are bathed in liquid 
supplied abundantly by the descending sap, to which botanists 
give the name of cambium. This last layer is of the utmost 
importance, for we can perceive that in the course of the 
second year, by the process of vegetation, this intermediate zone, 
which is situated between the cortical and ligneous system, and 
