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found at the ends of the large ones. They are composed of 

 elongated cells, the walls of which contain numerous thin spots 

 on their surface. So thin are these that water can pass through 

 them but absolutely nothing else, not the smallest particle of solid 

 material, except the root is in any way injured. These cells are 

 packed close together, and the thin spots in each coincide with 

 those of the adjacent one. There is no opening whatever in their 

 walls. So far, science has not actually proved exactly how mois- 

 ture ascends to the leaves of a tree. As many cells are found in 

 the stem that have such dense walls, it almost appears impossible 

 for moisture to penetrate them. And yet it is by these cells in 

 the stem that the sap ascends, the action being different to that of 

 the root cells. Whenever two liquids of different densities are 

 separated from each other by a membranous partition, it is na- 

 ture's law that the denser fluid v\?ill attract the lighter, until both 

 become the same density. This action is called endosmose. Now 

 the cells in the roots of trees contain reserve material, stored there 

 from the previous year's growth, and composed of mucilage and 

 protoplasm, which is far denser than water. When the soil gets 

 moist, the water in it is attracted through the cell walls to the 

 denser liquid in the cell. This action goes on from cell to cell 

 till it reaches the stem. Here capillary action starts. The sap 

 ascends to the leaves, through the wood cells of the vascular bun- 

 dles. These, in addition to other kinds of cells, form what is 

 known as the sap-wood. The action of the rising sap is very 

 rapid. As soon as it gets to the leaves it is elaborated and re- 

 verts again to the roots. The water taken up is largely evaporated 

 by the leaves ; the matter retained by them being the nourishment 

 obtained from the soil, plus the reserve material from the roots. 

 This at once causes a denseness of cell contents, that attracts the 

 thinner rising sap, and so the action goes on till want of moisture 

 in the soil prevents it. A tree, as is well known, is composed of 

 two kinds of wood — heart and sap. The heartwood is to all in- 

 tents and purposes dead, it takes no part in the life of the tree, 

 other than to support the crown. It is in the heart that decay first 

 sets in. It is possible for a tree to live for years with little or no 

 heartwood, as may be seen in the case of a hollow one. The 

 sapwood on the other hand, is the life of the tree, as by it the 

 sap ascends and descends. 



Having now seen how sap rises in a tree, the next and most 

 important point is how does it come down, for it does come down, 

 otherwise tree stems would never increase in girth. The leaves 

 are the organs of nutrition, as well as respiration. The substance 

 sent up by the roots has been utilized to form new shoots and 

 leaves, in other words, height growth. As soon as warm weather 

 sets in, the leaves begin to collect material from the atmosphere, 

 mostly, carbon. Quite four-fifths of the carbon used in the struc- 

 ture of a tree is obtained in this way, the quantity used may be 

 guaged by the amount of charcoal left after the wood has been 

 burned. They also return to the atmosphere oxygen. The pro- 

 cess of respiration is carried on through minute cells, found chiefly 



