ROOTS AND THEIR WORK 



87 



A 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 ? 



a little first-hand evidence as to its internal structure. If you 

 cut open a parsnip or carrot so as to make a cross section of the 

 root, you find two distinct areas, an outer portion, the cortex, and 

 an inner part, the central cylinder. 

 If you cut another parsnip in 

 lengthwise section, these struc- 

 tures show still 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 wav 

 out through the cortex wherever 

 the small rootlets are given off. 

 Make a drawing that will show 

 these points. 



Fine Structure of a Root. — If 

 we could now examine a much smaller 

 and more delicate root in thin longi- 

 tudinal section under the compound microscope, we should find the follow- 

 ing structure: (Cross sections and longitudinal sections of Tradescantia 

 roots are excellent for demonstration of these structures.) The entire root 

 is seen to be made up of cells, the walls of which are uniformly rather thin. 



The cells of this part of the root are 

 more or less regular in size and shape. 

 The central cylinder can easily be dis- 

 tinguished from the surrounding cortex. 

 The cells of the former have somewhat 

 thicker walls. In a longitudinal sec- 

 tion a series of tubelike structures may 

 be found within the central cylinder. 

 These structures are, in fact, cells which 

 have grown together at the small end, 

 the long axis of the cells running the 

 length of the main root. In their de- 

 velopment 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. 



Cross section of a young taproot; a, a, 

 root hairs; b, epidermis; c, cortical 

 layer; d, fibrovascular cylinder. 



