NATURE OF PLANTS 



47 



scarcely more than one millimeter in length (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 portion 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 elongating. The first 

 space from the tip has grown very little, the second and third 



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 appearancj 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. 



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 

 rap'dity that characterizes their formation near the tip (Fig. 

 29, B). Before the significance of these peculiar features can be 



