THE ROOT 



161 



as it forces its way through the more or less resistant soil. The 

 tissues of the root usually become stiffened at an early stage by 

 the rapid thickening of the walls of the tracheary elements of the 

 primary wood. Rigidity is so much a necessity in the case of the 

 wood of the root that ringed and spiral tracheids, such as ordinarily 

 characterize the first-formed region of the xylem in the aerial stem, 

 are conspicuous by their absence or at any rate by a scanty degree 

 of development. In Fig. 117 are represented the protoxylem of 

 the stem and root of the balsam fir. The great contrast in the 

 development of the extensible ringed and spiral tracheids is shown 



FIG. 117. Longitudinal view of the primary tracheids of the root and stem of 

 the balsam fir, showing the different degrees of development of the protoxylem ele- 

 ments in the two organs. 



in the diagram. It is obvious that ringed and spiral sculpture alone 

 is not an entirely constant character of the first-formed wood in roots. 

 A great deal of morphological importance has in the past been 

 attached to the arrangements of cells at the apices of the roots. 

 It is not clear, however, that inferences drawn from such data 

 have a very great evolutionary significance; and certainly in other 

 organs of the plant, such as, for example, the stem, they are of 

 very slight value in view of the extremely contradictory results 

 reached as a consequence of adherence to this criterion. At the 

 present time the organization of the growing point in plant organs 

 is regarded as of less importance than the histological structure 

 of the mature parts in reaching any conclusions as to equivalence 

 and course of evolution. 



