THE ANATOMY OF WOODY PLANTS 



in general in vascular plants. The illustration supplied by the 

 rays serves as an exemplification of the original condition of the 

 root as regards its secondary structure. Additional examples will 

 present themselves repeatedly in the subsequent chapters. In the 

 case of the primary organization the uniformity of the radial struc- 

 ture presented by the root throughout the vascular series, in contrast 



to the great variety of 

 types exemplified by the 

 stem both in regard to 

 the arrangement and 

 mode of development of 

 the fibrovascular ele- 

 ments, vouches even 

 more emphatically for 

 the value of the anat- 

 omy of this organ in 

 evolutionary investiga- 

 tions which are not 

 entirely speculative in 

 their nature. 



It will be convenient 

 and appropriate at this 

 stage to consider the 

 root in the herbaceous 

 dicotyledons. The 



Ranunculaceae will serve here as an excellent exemplification of the 

 conditions in plants in which the annual herbaceous habit has 

 brought about at once reduction in the development of the second- 

 ary tissues and marked modifications in their topography. Fig. 112 

 illustrates photographically the organization of a root in the case of 

 Actaea alba. The cortical tissues are somewhat clearly separated 

 from those of the fibrovascular system by an endodermal zone. The 

 central region of the root is distinguished by the presence of four 

 aggregates of phloem. A marked feature of the secondary wood is 

 its division into widely separated segments by broad rays of the com- 

 pound type. This condition is extremely common in roots of forms 

 in which the herbaceous habit has become strongly developed. 



FIG. 112. Transverse section of a root of 

 Actaea alba. 



