6o 



Introduction to Botany. 



conducting elements by the process of girdling, for the 

 reason that the bundles are scattered promiscuously 

 throughout the stem (Fig. 20). 



42. Summary of Structure and Function. — When we re- 

 view the details in the plan of construction of typical stems, 

 we see that they are so admirably adapted to give strength 



and facilitate transport that 

 trees can withstand the storms 

 of centuries, and the inter- 

 change of materials between 

 roots and leaves can continue 

 without interruption even in 

 trees which have reached a 

 height of four hundred feet. 

 We see that the strengthening 

 elements are laid down ac- 

 cording to approved mechan- 

 ical principles, and that the 

 conducting system is a double 

 highway along which mate- 

 rials may pass in opposite 

 directions without hindering 

 each other in the least degree. We find that the movement 

 of materials radially is provided for by the medullary rays, 

 which extend individually only a short distance longitu- 

 dinally, and so are prevented from transporting materials in 

 any other than the radial direction (see Fig. 22). 



43. Transporting Forces. — The forces which are con- 

 cerned in carrying the water upward have not yet been 

 demonstrated with certainty. Atmospheric pressure does 

 not suffice to carry water to the height of tall trees ; capil- 

 larity in the tracheal tubes cannot lift the water beyond 

 the height of a middle-sized tree ; and osmotic pressure in 



Fig. 22. 



Diagram showing medullary rays from 

 cross, tangential, and radial points of 

 view. The rays are seen at i and h. 

 After Hartig. 



