The Trees of Vermont 197 



rapid, differs in nature from the harder and denser wood formed later 

 in the season. The oaks, elms and hickories present clearly defined 

 layers. The two layers are called early or spring wood and late or 

 summer wood. The presence or absence of well-defined spring and 

 summer wood is an important factor in distinguishing and naming the 

 different woods ; in fact, the grain or figure of some kinds of lumber 

 depends entirely upon the difference between spring and summer wood. 



Radiating from the center of the trunk toward the bark, like the 

 spokes of a wheel, are slender but more or less distinct hues. These 

 are the medullary rays (i, fig. VIII). Medullary rays serve two func- 

 tions ; they are paths for the conduction of food materials between 

 bark and wood or wood and bark, and they bind the trunk together 

 from pith to bark. Woods dift"er in the size and number of medullary 

 rays present, the rays being inconspicuous in some and very prominent 

 in others. If an oak log be cut along any of its many radii, i. e., parallel 

 to one of the lines of the medullary rays, the beautiful "flakes" of the 

 quarter-sawed oak are exposed to view. In other trees, like the maples, 

 the medullary rays may be so small as to escape notice. But whether 

 prominent or inconspicuous, the medullary rays help largely to make 

 up the pattern which often readily and surely distinguishes one wood 

 from another. 



We have learned from our superficial inspection that a tree is not 

 a simple structure but a complex organism composed of several kinds 

 of tissues, each having its particular function in the life of the tree. 

 In order to continue our study further, it is necessary that we use a 

 hand lens or a compound microscope. If we examine the smoothly 

 cut end. of a piece of wood, or, better, a very thin shaving, we find that 

 it is seemingly made up of a network of holes or pores held together 

 by chains of solid tissue, in appearance resembling the chambers of a 

 honeycomb. All trees, all tissues composing trees, are made up of 

 millions of these units, just as a wall is built of bricks and mortar. 

 These units making up the bodies of trees are called cells (fig. IX). 

 A cell may be considered in the nature of a box, but so small is it that 

 it is usually indistinguishable to the naked eye. The walls of the box 

 or cell are composed for the most part of a woody substance called 

 cellulose ; we call them cell-walls. In young cells, such as we find in 

 the actively growing parts of trees, the whole cavity of the cell is 

 filled with a slimy substance called protoplasm. The protoplasm is 

 the living substance of the tree; where there is protoplasm, there is 

 life and growth. As the cells grow older and larger, the protoplasm 



