NUMBER AND DIMENSIONS OF ROOT-HAIRS 219 



ecological relations of the plant play a great part, as appears from the 

 circumstance that either type of absorbing tissue may be developed 

 within a single species, or even in one and the same individual, accord- 

 ing to the nature of its requirements for the time being. There are 

 quite a number of water-plants, such as Elodea canadensis, Nv/phar 

 luteum, Acorns calamus and Cicuta virosa, the roots of which produce 

 no root-hairs so long as they are immersed in water, while they develop 

 these structures in abundance on entering the soil. 



Fuller consideration must be given to the type of absorbing tissue 

 which is characterised by the constant presence of root-hairs. 11 ' The 

 greatest interest attaches to the hairs themselves, since these structures 

 are almost entirely responsible for the functions which are ascribed to 

 the whole absorbing tissue. Eeference has already been made, on more 

 than one occasion, to the fact that root-hairs owe their existence to the 

 principle which aims at the maximum exposure of surface. The 

 increase of absorbing surface which is effected by the development of 

 these structures is indeed very considerable. Schwarz has estimated 

 its numerical value in certain cases. His method consists in deter- 

 mining the number of root-hairs borne on 1 sq. mm. of root surface 

 and in measuring their average length and diameter ; the total surface 

 exposed per unit length can then be readily calculated. In this way 

 it was found, for example, that the surface of a Maize root growing 

 in a moist chamber, and consequently well provided with root-hairs, 

 exceeds the surface of an [imaginary] completely hairless root in the 

 ratio of 5'5 to 1. For roots of Pisum sativum, likewise grown in a 

 moist chamber, the corresponding ratio is 12 "4 to 1, for the aerial roots 

 of Scindapsus pinnatus 18"7 to 1, and for the lateral roots of Trianea 

 bogotensis grown in water 6*63 to 1. It must not, however, be 

 supposed that these figures indicate any fixed ratios peculiar to the 

 different species ; on the contrary the increase of surface effected by 

 the formation of root-hairs corresponds very closely to the relative 

 humidity of the soil, inasmuch as both the number and the 

 average length of the hairs are reduced in very wet soil, where water 

 and nutrient salts are available in abundance. This reduction is 

 carried to its logical conclusion, when the root-hairs are entirely sup- 

 pressed in contact with liquid water ; this condition, which occurs, 

 according to Schwarz, in Allium, Ccpa, Ciccr arietinum, Cucurbita Pepo, 

 Hclianthus annuus, Phascolns communis, P. multiflorus, Ricinus com- 

 munis, Zea Mays, etc., may be regarded as a reversion to the simpler 

 type of absorbing tissue. A low degree of humidity in the soil, on the 

 other hand, renders the absorption of water and nutrient materials 

 more difficult, and thus necessitates an increased production of root- 

 hairs. A very dry soil, finally, retards the appearance of the hairs and 



