I.] CAMBIUM. 35 



as oxygen, etc., obtained from the rain-water in the 

 soil and taken up by the root. This "sap" is found to 

 move upwards every spring, and continues for a time to 

 flow through the tubular and pipe-like structures com- 

 posing the wood of the tree until it reaches the leaves ; 

 here it is distributed to the cells containing the green 

 chlorophyll, and gives up to them its minerals. These 

 living leaf-cells, properly supplied with water and 

 mineral salts, and exposed to the sunlight, are able to 

 manufacture from the carbon dioxide of the air, certain 

 organic compounds which require very little chemical 

 change to become wood-substance. These compounds 

 are then carried down from the leaves into the stem, 

 and pass, by various routes — e.g. the medullary rays, 

 the inner tissues lining the bark, etc. — to wherever they 

 are needed by the growing parts of the tree. 



Of these growing and living parts of the stem none 

 is so important to us as the cambium, a very thin and 

 delicate layer of active cells, easily found in immediate 

 contact with the outside of the sap-wood, and often 

 regarded by the uninitiated as a slimy substance between 

 the wood and the inner bark. It is not a mere 

 substance, however, but a definite, though extremely 

 tenuous, mantle of living and growing cells, fed by the 

 substances dissolved in the sap handed on to it from 

 the leaves. 



This cambium, if properly supplied with food- 

 materials, adds new layers of wood on to the outside 

 of the wood already formed, and new layers of other 

 living tissues to the inside of the structures found 

 beneath the bark, which structures may be collectively 

 termed the cortex. 



The stem is thus enlarged periodically by a new 

 layer on the outside of the alburnum, and by the 



