68 How a Tree Lives 



The sap, containing the food a tree needs, passes 

 from the root hairs into pipes in the root. The pipes 

 run all over the tree and take the sap up the trunk, 

 along to the very tips of all the branches and into the 

 leaves. In the leaves these pipes form a complete 

 network of veins, so that no part of the leaf is left 

 without its supply of food and water. 



The leaves keep all the dissolved food and as much 

 of the water as they need. The rest, in the form of 

 invisible moisture, passes out through tiny openings in 

 the skin of the leaf. In the winter when the leaves 

 have fallen, trees, as you will see in a later lesson, get 

 rid of any moisture they may not want through the 

 bark. During the cold weather, however, the trees do 

 not take up nearly as much water as they do at other 

 times when they are actively growing. When the spring 

 comes they wake up into renewed growth and sap 

 passes once more freely up their trunks. If the branch 

 of a growing vine tree is cut in the spring, you will see 

 great drops of sap oozing from the cut end. This shows 

 you how strongly the sap was flowing along the branch. 



The sap, then, contains all the soil food which a 

 growing plant needs. If there were no water in which 

 this food could be dissolved a plant would in time 

 starve, were it not for the fact that long before this 

 time comes it would have withered away for want of 

 water. 



All the different parts of plants, stem, leaves, fruit, 

 etc. are made up of countless tiny boxes placed side by 

 side. The sap soaks through the walls of these boxes 

 or cells and keeps them swollen and hence firm. If 

 a young green plant is left many hours without water 

 the walls of these cells collapse and the plant hangs its 



