
SHAVINGS EXAMINED MICROSCOPICALLY. 567 
here it is brought in contact with the sunlight and air, and 
certain chemical changes take place in its composition. 
Downwards, through another set of ducts, it is carried just 
inside the bark, and here through its instrumentality, woody 
fibre is deposited, one fibre upon the other externally, and 
thus the twig grows by outside growth, becoming thicker 
and thicker each year. This addition of substance goes on 
during the spring and autumn months, the plant doing very 
` much the same as human beings, that is to say, resting dur- 
ing the hot season. But when winter comes its growth is 
arrested entirely, and like the hibernating animals the tree 
sleeps. Now in animals the blood is carried by a set of: 
vessels, known as arteries, to the lungs, where it comes in 
contact with the air inhaled, and has its composition so 
changed that it can build up new tissues. The same thing, 
essentially, we see, takes place in the tree, the leaves repre- 
senting the lungs, or oxygenating organs. Now as the tree 
sleeps during the winter months here is an arrest of growth, 
and therefore when we examine such a cross-section of a 
piece of wood as we have given, we find a number—less or 
greater, according to the number of winters it has existed — 
of these rings of arrested growth, and by counting them we 
can arrive at the age of such a stick of wood. So we see 
how the microscope assists in acquiring such a knowledge ; 
and of course we shall find similar structure in all outside 
growers or Hxogens. With inside growers the case is very 
different; for here the new matter is not deposited exter- 
nally in regular rings ; and, in fact we can, from a considera- 
tion of the facts we have related, readily understand why the 
Endogens are mostly confined to such portions of the globe | 
Where there are no cold months to arrest the growth. How- 
ever, even in such climates, Hxogens grow and rest also 
during a part of the year. We have given the two sections 
represented to show the very marked difference in these two 
modes of growth as illustrated by microscopic sections, and 
those who desire to verify our illustrations can readily do so 
