114 



BOTANY 



four tubes of different inside diameter, the fluid will be found to rise very 

 much higher in the tubes having a smaller diameter. This is caused by cap- 

 illarity or capillary attraction. When we consider that the tubes of the fibro- 

 vascular bundles are very much smaller than any we can make out of glass, 

 it can be seen that water might rise in the stem to some height in tubes of 

 microscopic diameter. 



Another suggested method for the rise of water is given in the fact that 

 air is found in some of the tubes in the form of bubbles, and these minute 

 bubbles may help in the ascent of water. 



The greatest factor, however, is one which will be more fully explained 

 when we study the work of the leaf. Leaves pass off an immense quan- 

 tity of water by 

 evaporating it 

 in the form of 

 vapor. This 

 evaporation 

 seems to result 

 in a kind of suc- 

 tion on the col- 

 umn of water 

 in the stem. In 

 the fall, after 

 the leaves have 

 gone, much less 

 water is taken in 

 by roots, show- 

 ing that an inti- 

 mate relation 

 exists between 

 the leaves and 

 the root. 



Structure 

 of Wood. — 



Quite a differ- 

 ence in color 

 and structure 



is often seen between the heart wood, composed of the dead walls 

 of cells occupying the central part of the tree trunk, and the sap 

 wood, the living part of the stem. In trees which are cut down 

 for use as lumber and in the manufacture of various furniture, 

 the markings and differences in color are not always easy to 

 understand. 



Cross section through a black oak showing heart wood and sap wood 

 and medullary rays. (From Pinchot, U.S. Dept. of Agr.) 



