158 



THE ANATOMY OF WOODY PLANTS 



In the monocotyledons the root is characterized by the usual 

 absence of any indications of secondary growth. In Fig. 114 is 

 represented the transverse view of the root of the carrion plant, 

 Smilax herbacea. The root hairs are very conspicuous (Fig. 115), 

 and underneath the layer which forms them, the piliferous layer, is 

 seen a zone of cells which, with certain exceptions, possess thickened 

 walls. Next to this structure, known as the exodermis, lies the 



cortex, which terminates 

 with another cellular 

 limiting membrane, the 

 endodermis. The center 

 of the root is occupied 

 by the central cylinder 

 and the pith. The for- 

 mer consists of numer- 

 ous and distinctly 

 alternating clusters of 

 primary xylem and 

 phloem which, as is 

 usual in roots, occupy 

 different radii. There 

 is no indication of 

 secondary thickening in 

 connection with either the xylem or the phloem. The wood 

 elements are smaller on the outside and become larger as they 

 pass inward. Although the usual gradation in size is found to 

 exist in the elements of the wood in monocotyledonous roots, the 

 order of development of the tracheary cells is not infrequently 

 the reverse of their gradation in diameter. In other words, 

 the internal elements of a tracheary nature are completed before 

 those lying farther outward in the region of the protoxylem. This 

 is one of the many abnormalities which characterize the anatomical 

 structure of this important group of the angiosperms. The central 

 region of the root is occupied by a well-marked pith, a condition 

 of very general occurrence in the group under discussion. A 

 similar situation in regard to the frequent presence of medullary 

 structures is found in the case of herbaceous dicotyledons. It seems 



FIG. 114. Root of Smilax herbacea 



