ANNUAL MEETING. 163 



called the pores of the wood. Running from bark to pith are tiie medullary 

 rays. These are thin vertical plates in tlio wood cells, and constitute tlie silver 

 grain of oak, maple, and other trees. The outer portion of the wood and the 

 inner bark constitute tlie cambium layer. Every springy this cambium 

 expands by a multiplication of cells, and makes a new layer of wood outside of 

 the previous wood, and a new layer of bark inside of the previous bark. The 

 layers of wood remain where they grew, the youngest and most active layers 

 on the outside. The bark cannot remain stationary, but must give way for 

 the enlarging trunk. In this way the bark stretches and cracks, and assumes 

 a variety of forms peculiar to different kinds of trees. Outside of the cambium 

 comes the liber made of short cells, with walls not very thick. Next comes 

 the bast tissue, which is composed of very long cells, with walls which are very 

 thick, giving rise to the toughness of the bark. Next comes the layer of thin 

 •cells, made green by the chlorophyl they contain. Then comes the corky 

 layer, and last the epidermis. The epidermis is glossy as if waxed, as it really 

 is in many places, to render it impervious to water. This is briefly the struc- 

 ture of a stem of the box-elder one year old. 



In some trees the green layer near the outside continues to live and produces 

 a layer of cork every year. In case of the cork oak of Europe, this is removed 

 without damage to the tree every few years. In summer the bark can be 

 peeled, as we say, because the cells of the new layer of wood are yet very thin 

 and delicate. Some of them are ruptured and destroyed as the bark peels. 



An internode of a young stem elongates for all or a good portion of its 

 length as it is developing. A stem has a pith. A root has no pith, and 

 •elongates at the tip only. These are two of the leading distinctions between 

 a root and a stem. Each usually adds a ring or layer of wood every year. In 

 hot climates these rings are often very uncertain, and are unreliable for 

 determining the age of a tree. 



In hard wood the cell walls are thick ; in soft wood they are thin. 

 In our climate where trees have a period of rest, it is usually very easy to 

 tell the age of a tree by counting the layers of growth. In the early part of 

 ■the season when growth is rapid, there are usually more of the large cells 

 formed — the cells which make the vessels. Later in the summer the cells are 

 smaller and do not, as a rule, contain any ducts or vessels. 



The wood of a tree may be compared to a lot of hollow cones set over each 

 other, the larger one covering all the rest. The cone consists of one layer 

 only on the twigs of one year's growth. In a thorn the cone is quite tapering; 

 in a twig of willow it is almost a cylinder. 



After a time, varying with the individual tree and the kind of tree, some of 

 the new or sap-wood changes its color and becomes heart-wood. The outer 

 sap-wood is alive and helps the tree convey sap. The heart-wood is generally 

 destitute of vitality, and so far as the good of the plant is concerned except to 

 answer as a support, it might as well be taken away. 



In autumn the leaves of our deciduous trees perish, and in most cases soon 

 drop to the ground. Before perishing, much of ihe nourishment of leaves goes 

 into the buds and branches. At this time the root hairs and many of the 

 smallest roots also perish. There is a wonderful contraction of surface below 

 ground as well as above ground when a deciduous tree prepares for winter. 



Evergreens hold their leaves for more than one year; some for two years; 

 some for three, four, five, six, seven, or more; but every autumn, or once a 

 jear regularly, one crop of leaves dies and falls. Deciduous trees are of a 



