46 



NATURE OF ROOT HAIRS 



are extremely delicate tubular outgrowths of the epidermal cells 

 (Fig. 30). Note that they always begin to grow a few mm. 

 back from the root tip and at the opposite end of the zone of root 

 hairs the tubes are withering or dying off, leaving the older por- 

 tion of the root covered only with the epidermis. If a root is 

 marked into 2 mm. spaces with India ink it will be found after 

 12 to 15 hours that only the portion below the root hairs is elonga- 



FiG. 29. Fig. 30. 



Fig. 29. The absorbing surface of roots: A, seedling of mustard showing 

 extent of surface covered by root hairs. B, an older branching root, the 

 shaded areas near the tip are due to particles of earth clinging to the root hairs. 

 The remaining portions of the root are free hairs and take little or no part in 

 absorption. 



Fig. 30. Origin and character of root hairs: A, first appearance of a root 

 hair due to the pushing out or growth of a definite portion of the surface of an 

 epidermal cell. B, older hairs that have become irregular through coming 

 in contact with soil particles. 



ting. The first space from the tip has grown very little, the 

 second and third much more, and in the other spaces there is a 

 gradual retarding of the growth until 8 to 12 mm. from the tip 

 elongation has ceased. It is at the point where elongation has 

 ceased that the root hairs begin to appear, but the total number 

 existing on the root at any one time does not materially vary 

 owing to the fact that they are dying off farther back on the 

 root with the same rapidity that characterizes their formation 

 near the tip (Fig. 29, B). Before the significance of these pe- 

 culiar features can be understood it will be necessary to con- 



