160 THE GROWTH OF ROOTS AND STEMS. 



produce new cells. Most of these cells go to form the main 

 mass of the root, but those formed towards the tip of the 

 root add to the root-cap. The root-cap is, therefore, con- 

 tinually renewed from within as it wears away outside ; 

 compare with this the way in which one's skin is renewed. 

 The onward growth of the root is due to the formation 

 of new cells at the growing-point and to their elongation 

 (and further division) behind the growing-point. The grow- 

 ing-point is not only the factory where new cells are made ; 

 it is also the sensitive part of the root, which alone can per- 

 ceive the stimuli of gravitation, etc., and by sending them 

 on to the region behind cause the bending movements. 



Root -'cap 



Fig. 58. The Root-Tip, in Section, magnified. 



Examine with the microscope the roots of any small seedlings (Cress, 

 Mustard, Wheat) mounted in water on a slide. Focus on the upper 

 surface, and notice the outer layer of cells (skin-layer or epidermis) 

 immediately behind the extreme tip. Sketch part of this, then focus 

 deeper so as to get an "optical section" of the growing-point, and try 

 to make out the tissues as shown in Fig. 53, the root-cap covering the 

 denser layers of cells which form the growing-point, and which 

 gradually pass backwards into the central cylinder, the rind (cortex), 

 and the epidermis. 



Trace the origin of root-hairs, each of which is a long slender tube, 

 closed at the free end ; each hair arises as an out-growth from a single 

 cell of the skin-layer. Farther behind the root-tip the root-hairs are 

 longer, but they are absent from the oldest parts of the root i.e. those 

 nearest the stem. Crush a root by pressing it under the cover-glass, 



