182 



BOTANY 



Fig. 172. — Tip of a root-hair with adhering 

 particles of soil. ( x circa 240.) 



ence in transplanting or for pot culture, and also to force a more vigorous develop- 

 ment of the lateral roots. 



Desert or xerophilous plants, according to the observations of Yolkexs, send 

 out deeply penetrating roots, which only branch profusely on reaching depths 

 where they find water. 



In order to secure a still more intimate contact with the particles 



of the soil, there are produced from 

 the surface of roots small, exceed- 

 ingly numerous and fine, cylindri- 

 cal bodies, which penetrate the 

 smallest interstices of the soil, and 

 fasten themselves so closely to its 

 smallest particles as to seem actu- 

 ally grown to them (Fig. 172). 

 These ultimate branches of the root system, 

 which discover the very smallest quantity 

 of moisture, and seek out the most con- 

 cealed crevices in their search for nourish- 

 ment, are the root-hairs (p. 95), — delicate 

 tubular outgrowths of the epidermal cells. 

 Although they have the diameter of only 

 a medium -sized cell, and appear to the 

 naked eye as fine, scarcely visible, glistening 

 lines, they often attain a length of several 

 millimetres and enormously enlarge the ab- 

 sorbing surface of their parent root. Accord- 

 ing to F. Schwarz the epidermal surface of 

 the piliferous zone of the roots of Pimm, 

 which has 230 root-hairs to the square milli- 

 metre, is thus increased twelvefold. 



The root-hairs do not cover the whole 

 surface of roots, not even in the youngest roots, 

 but only a comparatively small zone, a short 

 distance above the growing root-tip. Soon after 

 they have attained their greatest length, and 

 have come into the closest contact with the 

 earth particles, they die off. New root-hairs are 

 developed to supply their place, so that a zone 

 of root-hairs is thus constantly maintained just 

 above the root-tip ; while beyond this advancing 

 zone of hairs the root epidermis becomes again Fig. 173. — Seedling of Carpfmms 

 completely divested of root-hairs (Fig. 173). * etulus - ''• ?°™ ° f /° ot - 



r J . . ... „ . . i hairs near root-tip ; A, hypo- 



To be convinced of this fact, it is only necessary C otyi; lur, main root; sro, 

 to carefully pull up a young plantlet growing in lateral roots ; i, v, leaf ; c, 

 a loose and not too dry soil, as such a condition epicotyl ; '• cot ^ Aons - 

 is especially favourable for the development of root-hairs. Each root, 



