THE TRACHEIDS 



107 



is composed entirely of fiber-tracheids which serve perfectly 

 well for the conduction of water, even to the tops of tall trees. 

 It does not necessarily follow, however, that tracheids are as 

 efficient in the transport of water as are tracheal tubes, for under 

 like conditions the leaves of pine transpire very little water com- 

 pared with the leaves of ordinary dicotyledonous trees. The 

 leaves of the beech, for example, weight for weight, give off 

 ten times as much water as do those of the pine. The pine tree 

 relies upon these fiber-tracheids for strength as well as for the 

 conduction of water, and the success of the plan is shown by 

 the fact that pines grow to be great trees and thrive in exposed 

 situations where the stress of the winds and demands for water 

 are heavy. The essential details leading to the success of this 

 plan appear to be these: First, as to water conduction, practi- 

 cally the whole of the early growth is devoted to this, and 

 the late growth materially assists, and the xerophytic leaves 

 keep transpiration down to a minimum. Second, as to 

 strength, the thin places in the tracheids are reenforced 

 by prominently overhanging borders, and the two weakest tissues 



FIG. 51. Different stages in the development of a bordered pit. b, The original, thin, 

 primary wall; a, the overhanging border formed as the wall thickened. .B.VThickening of 

 the wall has continued and extended the border; the primary wall has thickened at c, 

 forming the torus. C, the border and the torus are finished. 



in wood, namely the wood parenchyma and the tracheal tubes, 

 are, respectively, almost and entirely lacking. Considered as 

 strengthening elements the fiber-tracheids are in all essential 

 respects like wood fibers. They are long and tapering cells, 

 their ends interlace, their walls are thickened and lignified. 



