THE ROOT 



153 



the secondary roots definitely take their origin from the same 

 clusters of protoxylem which are subtended by aggregate rays in 

 the secondary wood, it follows that the secondary roots are corre- 

 lated with aggregate rays precisely as the traces of the leaf are 

 imbedded in similar modifications of the radial parenchyma in 

 the case of the stem. Aggregate rays are much more commonly 

 found in the root than they are in the shoot organs, a consequence 

 of the conserv- 

 atism of the root, 

 which is to be 

 emphasized in a 

 later chapter. The 

 intimacy of the re- 

 lation between the 

 aggregate ray and 

 the trace of the 

 secondary root can 

 be readily inferred 

 from the inspec- 

 tion of Fig. 109, a 

 photograph of a 

 transverse section 

 of the root oiAlnus 

 japonica. It is 

 clear that the trace 

 is related to a mass 

 of enlarged rays among which vessels are conspicuous by their 

 absence, precisely as in the leaf trace illustrated in Fig. 130, 

 page 177. It thus becomes clear that aggregate rays may be 

 a feature of organization of the root as they are of the stem, and 

 that in both organs the clusters or congeries of rays are most 

 conspicuously developed in relation to the appendages. 



The secondary structures of the root are characterized by the 

 same three types of multiseriate rays found in the wood of 

 the stem as figured and diagrammed in chapter vi. In the case 

 of the root, however, the primitive aggregate condition which pre- 

 cedes both the compound and diffuse types tends strongly to persist. 



FIG. 109. Transverse section of part of same, more 

 highly magnified. Description of this and last figure in 

 the text. 



