24 AGRICULTURE. 



CHAPTER VI. 



STRUCTURE AND GROWTH OF THE PLANT. 



THE SAP. All the water used by the plant enters through the 

 roots, and along with it comes the material that we call the 

 mineral matter, together with the nitrogen that the plants require. 

 The stales and branches form the frame work of the plant its 

 body, so to speak. The leaves give off the water taken in by 

 the roots, and also take up carbon from the carbonic acid gas of 

 the air. Now if the water goes in by the roots and out from the 

 leaves it must move through the plant through the roots to the 

 stalk, thence to the branches, and so on to the leaves. This 

 water contains many substances in solution (sugar, salts, and 

 other things) ; we call it sap, and the movement is called " the 

 circulation of the sap." We have already referred to the fact 

 that a limb will split lengthwise, not across. Sometimes, as in 

 flax and in the inner bark of basswood, we can pull off long 

 fine strings of fibre. These long fibres that run up and down, 

 or lengthwise, are nothing else than strings of little cells, and 

 in circulation the sap passes on through from one to the next. 



Frequently you see a hollow tree that is alive and thrifty 

 and when you cut across a large tree you notice that the sap is 

 principally in the outer portion. The outer rings of wood are 

 much wetter than the inner or heart wood. We conclude, 

 then, that the sap moves principally up and down through the 

 layers or fibres of the plant near the outside, just under the 

 outer rough bark. The life of the body of the plant is then 

 mainly near the outer bark. When we girdle a tree we are apt 

 to kill it ; we can cut a small nick into it, we can tap it, or we 

 may bruise a piece of the bark, and we do not kill it. Now 

 you see the reason. 



