ROOTS 43 



(c) The cortical portion, a tubular part enclosing the solid 



central cylinder. 



(d) The root-hairs, wnich cover some parts of the outer layer of 



the cortical portion very thickly. Observe particularly 

 how far toward the tip of the root the root-hairs extend, 

 and where the youngest ones are found. 



Make a drawing to illustrate all the points above suggested 

 (a, b, c, d). Compare your drawing with Fig. 20. Make a careful 

 study of longitudinal sections through the centers of the tips of very 

 young roots of the hyacinth or the Chinese sacred lily. Sketch 

 one section and compare the sketch with Fig. 20. 



Make a study of the roots of any of the common duckweeds, 

 growing in nutrient solution in a jar of water under a bell-glass, and 

 note the curious root-pockets which here take the place of root-caps. 



54. Details of Root-Structure. The plan on which the 

 young root is built has been outlined in Sect. 53. A few 

 further particulars are necessary to an understanding of 

 how the root does its work. On examining Fig. 21 the 

 cylinders of which the root is made up are easily dis- 

 tinguished, and the main constituent parts of each can be 

 made out without much trouble. The epidermis-cells are 

 seen to be somewhat brick-shaped, many of them provided 

 with extensions into root-hairs. Inside the epidermis lie 

 several layers of rather globular, thin-walled cells, and 

 inside these a boundary layer between the cortical or bark 

 portion of the root and the central cylinder. This latter 

 region is especially marked by the presence of certain 

 groups of cells, shown at w and d and at 5, the two 

 former serving as channels for air and water, the latter 

 (and w also) giving toughness to the root. 



Roots of shrubs and trees more than a year old will 

 be found to have increased in thickness by the process 



