ROOTS AM) THEIR WORK 



87 



more plainly. An additional fact is seen; namely, that all the 

 smaller roots leaving the main or primary root have a core of wood 

 which bores its way out through 

 the cortex wherever the small 

 rootlets are given off. 



Fine Structure of a Root. If 

 we could now examine a much 

 smaller and more delicate root in 

 thin longitudinal section under 

 the compound microscope, we 

 should find the entire root to 

 be made up of cells, the walls A 

 of which are uniformly rather 

 thin. (Cross sections and lon- 

 gitudinal sections of tradescantia 

 roots are excellent for demonstra- 

 tion of these structures.) Over 

 the lower end of the root is 



cross section through a taproot (a 

 parsnip) : C, cortex; W, wood. Notice 

 in the right-hand specimen, which 

 has been dipped in iodine, that the 

 core of wood continues out into the 

 rootlets which leave the main root. 

 Where is most starchy food stored in 

 a parsnip? 



found a collection of cells, most of which are dead, loosely ar- 

 ranged so as to form a cap over the growing tip. This is 

 evidently an adaptation which protects the young and actively 

 growing cells just under the root cap. In the body of the root 

 the central cylinder can easily be distinguished from the surround- 

 ing cortex. The cells of the former have somewhat thicker walls. 



In a longitudinal section a series of 

 tubelike structures may be found 

 within the central cylinder. These 

 structures are cells which have grown 

 together at the small end, the long 

 axis of the cells running the length 

 of the main root. In their develop- 

 ment the cells mentioned have grown 

 together in such a manner as to lose their small ends, and now form 

 continuous hollow tubes with rather strong walls. Other cells 

 have come to develop greatly thickened walls ; these cells give 

 mechanical support to the tubelike cells. Collections of such tubes 

 and supporting woody cells together make up what is known as 

 fibrovascidar bundles. 



The end of a growing root, tipped 

 and protected by the root cap ; 

 g, the growing point. (Consider- 

 ably magnified.) 



