Stem and Root Structures 



79 



Phloem 



Fig. 76. — Root of carrot, cross section. 

 c, Cork ; cp, secondary parenchyma ; 

 sx, secondary xylem. 



roots extending in four rows nearly its entire length. At the 

 base of each root a slender 

 horizontal slit is seen. 



A cut across the carrot 

 shows a circle of tissue 

 about midway between 

 centre and circumference ; 

 this region is actively divid- 

 ing and adding to the thick- 

 ness of the root. A carrot 

 which has been left for a 

 time in methyl blue will 

 show traces of blue in the 

 part enclosed within this circle which indicates, as in the stem, 

 the region of wood vessels. They are surrounded by an abund- 

 ance of storage tissue of which the carrot largely consists. 

 Although the carrot has been immersed, a layer of cork pre- 

 vents the surface of the carrot from absorbing the solution. 



If a cross section is stained with iodine, the medullary rays 

 may be traced by the starch they contain. This is most abun- 

 dant outside of the active ring where the rays extend through 

 the phloem. 



Either a cross section or one cut lengthwise shows the 

 lateral roots extending to the central portion of the root. Un- 

 like leaves and branches, they arise in deep seated tissue and 

 make their way through the cortex of the main root. 



While points of resemblance may be seen between a stem 

 and a root, they show in their general form essential points of 

 difference. 



1. A root is protected by a root cap which is constantly 

 worn away as it pushes its way through the soil. A stem is 

 protected by the young leaves which form the bud. 



2. Roots give rise to roots, stems produce stems and also, 

 unlike members, leaves. 



3. Lateral roots arise within deep-seated tissue (endogen- 

 ously) ; lateral branches and leaves arise from surface tissues 

 (exogenously). 



4. The stele in roots is relatively small, that is, the rigid 



