PRINCIPLES OF THE GROWTH OF TREES. .3 



hundredth part of an inch in diameter, and in many plants 

 they are still smaller. Fig. i exhibits a section of the wood 

 of the maple cut across presenting many thousands of these 

 little vessels, only visible under a good microscope. The 

 branch of an apple-tree, an inch in diameter, cut across, shows 

 about one million. This cellular structure exists throughout 

 the roots, stems, shoots, leaves, flowers, and fruit. 



The cells of plants usually vary from i-3ooth to i-5ooth of an 

 inch in diameter, and it is obvious that during vigorous 

 growth the plant forms them with great rapidity. A shoot of 



FIG. i. Cross Section of Tree Trunk. 



asparagus increases the length of one cell every ten seconds; 

 and as its diameter embraces many thousands, from fifty to a 

 hunred million are formed every day. The building up of the 

 plant of these cells has been compared to the erection of a 

 house by the successive addition of bricks; but if as many 

 bricks were daily added to a structure, they would be enough 

 to make a building daily larger than the great pyramid of 

 Egypt, or the Coliseum at Rome. Yet every one of these 

 cells is as perfect and finished as the finest work of art. 



THE ROOT. 



The root consists of several parts (Fig. 2). The main root, 

 called also the tap root, is the large central portion, extend- 

 ing downward. In many plants or trees, however, it is 



