8 



PRACTICAL BOTANY 



until the roots are extremely small. During periods of active 

 growth root hairs appear upon the smaller rootlets (Fig. 5). 

 These rootlets, like the other parts of 

 the plant, are made up of many cells 

 (Fig. 6). Each cell has a wall, the cell 

 wall, which incloses the living mate- 

 rial, called the protoplasm. In the 

 root hairs, as in Fig. 6, two parts of 

 the protoplasm are shown, the nucleus 

 and the granular cytoplasm. Cells con- 

 tain other protoplasmic bodies, which 

 need not be discussed at this time. 

 The root hairs are extensions of 

 the surface or epidermal cells of the 

 rootlet and are parts of those cells. 

 They grow a little way back from 

 the tip of the rootlet and new ones 

 appear as the root tip pushes for 

 ward in the soil, so that with the 

 dying of older root hairs and the de- Note the diff erence in length and 



velopment of newer condition of the root hairs on the 



ones on new growth different parts of the root 

 of the rootlet, the actual number of root hairs 

 n on a rootlet during 



the growing season 

 may remain practi- 

 cally constant. It is 

 evident that the area 

 of root hairs on a 

 rootlet advances, al- 

 though the single root 

 hairs do not move f OI'- 



FIG. 5. A mustard seedling 



grown in a band of filter paper 



inside a drinking glass so as to 



show the root hairs 



w 



FIG. 6. Cells from the surface of a young rootlet 

 Showing epidermal cells (e), and one young and two 

 older root hairs (A). In the root hairs the nucleus 

 (n) and granular cytoplasm of the cells are shown. 



Greatly magnified 



laterally from the rootlet, growing through the soft particles 

 of the soil and around the harder ones, thus constituting a 



The root hairs extend 



