ROOT-HAIRS. 231 



620. Root-hairs. It was shown experimentally by Ohlert 1 in 

 1837, that the tip of the root is not the absorbing part. By 

 careful excision of the tip, and the use of a harmless water- 

 proof varnish to cover the wound caused, he obtained full absorp- 

 tion of liquids through the sides, and not the end of a young 

 root. He further demonstrated the very general occurrence of 

 delicate hairs upon the sides of young roots, and expressed the 

 opinion that these were the efficient agents in root absorption. 



621. That the abundance of the hairs on new roots is depend- 

 ent largely on the amount of moisture to which they are ex- 

 posed, appears from experiments on the roots of some of the 

 more common cultivated plants, Allium Cepa, Cucurbita Pepo, 

 Zea Mais, etc. In all these cases the plant can almost be said 

 to regulate the amount of its absorbing surface by the amount 

 of moisture within its reach, and it is thought by some that all 

 the epidermal cells of a young and developing root have the 

 power of extending into hairs. The number of hairs to the 

 square millimeter on a root of Zea Mais grown in a moist place 

 was found by Schwarz to be 425 ; and on a root of Pisum 

 sativum, 232*. 



622. Root-hairs are, as has been shown in Part I., cylindrical 

 protuberances from the external wall of the epidermal cells. 

 They vary in length from .1 mm. to 8 mm. The former length 

 occurs in a few grasses, the latter in some water plants. Schwarz 

 gives the following measurements of length : root-hairs of Pota- 

 mogeton, 5 mm. ; of Anacharis, 4 mm. ; of Brassica Napus in 

 moist air, 3 mm. ; of Pisum sativum and Avena sativa, 2.5 mm. ; 

 of Vicia Faba, 8 mm. 



623. When root-hairs are developed in contact with soil, they 

 become much distorted (see Fig. 89), and generally dwarfed; 

 they curve more or less irregularly around the particles of soil, 

 and frequently are enlarged at the immediate place of contact. 

 Moreover, the character of the cell-wall is somewhat changed at 

 the place of contact with the particles ; in many instances the 

 wall undergoes a sort of mucilaginous modification, and becomes 

 so firmly united to the particles that these cannot be removed 



1'hygroscopicite. Ces deux proprietes de tissu peuvent bien expliquer I'enorme 

 quantite d'eau qni penetre dans la plante vivante, les variations de cette quan- 

 tite selou les especes, les saisons, etc. II so'uffit d'admettre que les cellules des 

 spongioles donees de contractions alternatives, augmentent et diminuent alter- 

 nativement les meats intercellulaires, et tendent ainsi a absorber de 1'eau en 

 quantite proportionnee a la force et a la rapidite de leurs contractions vitales. " 

 1 Linnaea, 1837. 



