\ 



SHAVINGS EXAMINED MICROSCOPICALLY. 567 



ere 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 

 insidcKthe 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 Exogens. 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, Exogens 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 



/ 



