8 



PEACTICAL BOTANY 



until the roots are extremely small. During periods of active 

 growtli 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 eon- 

 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- 

 velopment of newer 

 ones on new growth 



of the rootlet, the actual number of root hairs 



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- 

 Fio. 6. Cells from the surface of- a young rootlet though the siagle root 

 Showing epidermal cells (c) , and one young and two h a i rs H n n nt mmro f r^. 

 older root hairs (h). In the root hairs thi nucleus ^ , "°* ™°^*^ *°^" 



' (n) and granular cytoplasm of the cells are shown Ward tllTOUgh the Soil. 



Greatly magnified The root hairs extend 



laterally from the rootlet, growing through the soft particles 

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



Fig. 5. A mustard seedling 



grown in a band of filter paper 



inside a drinking glass so as to 



show the root hairs 



Note the difference in length and 



condition of the root hairs on the 



different parts of the root 



