ROOTS THEIR STRUCTURE AND FUNCTION 51 



cells, the cortex. This is separated by a boundary layer from the 

 central cylinder which occupies the remainder of the root. 



In this central cylinder there are three sorts of tissues which 

 are also found in stems, though differently arranged. They are: 

 (1) wood and ducts, (2) bast, (3) cambium. 



The woody tissue is composed of thick-walled, hard cells which 

 give strength to the root but carry little sap, and ducts, which are 

 long, tubular cells, used principally for the transfer of sap upwards 

 toward the stem. 



The bast tissue consists of tough, fibrous cells, interspersed 

 with tubular ones. Its function is both to give toughness and to 

 carry sap downwards. 



The cambium is the most remarkable tissue in the plant. It 

 consists of thin-walled, very active cells, full of living protoplasm 

 which have the power of rapid growth. In fact, all growth of 

 the plant occurs here, and if the cambium be destroyed, the plant 

 will die. 



Since these tissues extend into the stem, where we will hear of 

 them again when we study stem structure, it is important that we 

 should remember their function in the root. 



The wood and ducts are generally grouped in four areas near the 

 center, and alternating with them, though outside, is found the 

 bast. The cambium forms a more or less complete ring between 

 the two. This arrangement permits the soil water to reach the 

 ducts without mixing with the digested food brought down from 

 the leaves by the bast. 



Around the tip of each growing root and extending up a little 

 way along each side is the protective root cap, composed of loose 

 cells easily rubbed off without allowing injury to the sensitive tip 

 as it pushes through the soil. The region of most active growth, 

 being back of this cap, is protected from injury, as would not be 

 the case if located at the extreme tip. 



Function of Root Parts. The function of the epidermis and its 

 root hairs is mainly absorptive. The cortex absorbs, retains, and 

 transfers the soil water; the ducts and bast tubes transfer liquids 

 and air, while fibers in both bast and wood give toughness and 



