152 



All Food Is Made by Green Flams unit hi 



Epidermis 

 Cortex 



xylem 



cambium 



phloem 



Growing point 

 Root cap 



Fig. 175 The tip of a root. Note the position of 



the xylan, the phloe?n, the caiiibimn, the cortex, 

 the epidermis, and the root cap. 



center is the conducting or vascular cyl- 

 inder. In the carrot the cortex is relatively 

 thicker than in most roots; most of the 

 food accumulates there. 



The cortex originally consists of thin- 

 walled, closely packed, more or less 

 rounded cells. Later there may develop 

 in it various thicker-walled cells which 

 give strength to the root. Also, cork tis- 

 sues form just beneath the epidermis. 

 The outer cork cells die, cutting off the 

 epidermis from contact with other living 

 cells of the root, killing it. Most cork 

 tissues are quite impermeable to water, 

 so water can enter only the youne^er 

 portions of roots in which the cork has 

 not yet formed. The young portion of 

 roots is near the tip where the root grows 

 in length. This is where the root hairs 

 form. 



Fig. 176 In cutting across a yoimg root one can 

 recognize three cylinders. IS! ame them in order 

 begijming on the outside. In very young roots 

 the xyle?fi cylinder is fluted. 



The vascular cylinder has various 

 kinds of cells. Near its center, in most 

 cases, there are water-conducting tubes. 

 These tubes form from cells that lie end 

 to end through the length of the root. 

 The cross walls that originally separated 

 each cell from those above and below it 

 disappear in time. Spiral thickenings 

 form lengthwise in the walls, and then 

 the protoplasm within disappears. Thus 

 long, thick-walled tubes are built up. 

 (See C in Fig. 180, page 155.) These 

 tubes vary in length from a few inches 

 to several yards, depending on how 

 many cells have joined together. Exer- 

 cise 7 may help you remember the vas- 

 cular cylinder. 



Among the tubes usually lie ivood 

 fibers. Each fiber is a long slender cell 

 which loses its protoplasm; its w^alls 

 chantTc into an elastic, tough, woody 

 material. The fibers give great strength 

 to the root. These fibers, together with 

 the tubes, are spoken of as xylem (zye'- 

 lem) the Crock Mord for wood. The 

 xylem makes up the inner portion of the 

 vascular cylinder. 



