THE FLOW OF SAP THROUGH THE STEMS 451 



the wood. The tough fibrous section of the bark is made up of 

 strong threads or fibers which run lengthwise down the stem. 



When we examine wood from which the bark has just been 

 removed, we find that it is covered with slimy matter. Between 

 the wood and the fibrous bark 

 is the cambium, a layer of deli- 

 cate cells too small to be seen 

 with the naked eye. The cells 

 of the bark are tough, but those 

 of the cambium are tender and 

 easily torn, and when the bark 

 is pulled from the stem, the 

 cambium breaks and slimy 

 matter oozes out of the cells 

 and spreads over the wood. FlG - 318. A cross section of a twig, 



^, ... showing bark (e), wood (w), and pith (p). 



Ihe central section or pith 



is a mass of soft dry matter, formed of cells which have dried 

 up, and are no longer of use to the plant. 



The flow of sap through the stems. You can see how soil 

 food flows upward through the stem by placing young stems of 

 lilac or other plants in red ink and leaving them there for a day. 

 When you remove the stems and split them in halves you 

 see that the ink has risen through the woody section close 

 to the cambium. It is not easy to show how the food sap 

 flows downward, but it has been proved that it passes into 

 the fibrous bark and makes its way through it to the roots. 

 If the fibrous bark of a tree is injured by deep cutting, the 

 downward flow of sap is stopped and the roots suffer for lack of 

 nourishment. The poorly nourished roots are unable to grow 

 or to take in sufficient soil moisture. The upward current of 

 water is small, the leaves droop from lack of water, and the tree 

 dies of slow starvation (Fig. 319). 



Girdling a tree is nearly always unwise, because although the 



CLARK INTRO. TO SC. 29 



