156 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



The formation of root-hairs is also influenced by the external con- 

 ditions. The most marked development of root-hair takes place on roots 

 grown in air saturated with water-vapour, or in moderately dry soil 1 . 

 The roots of Zea mays, Cucurbita pepo, Acorus calamus, form root-hairs 

 in soil, but in a normal water-culture they may be entirely or partly 

 absent. A lack of moisture, as well as too high concentration of the water 

 in the soil and too great mechanical resistance, hinder the formation of 

 root-hairs 2 . Aerial roots form root-hairs at the points of contact with 

 soil or damp walls because more moisture is present here, and not owing 

 to the absence of light, or to the action of a contact-stimulus, at least in 

 those cases which have been closely investigated 3 . The further growth of 

 the root-hairs is influenced by contact, as might be expected. 



We shall see later (Sect. 65) that the mycorhizae, formed on the 

 roots of forest trees by symbiotic union with fungal hyphae, act in a 

 similar manner to root-hairs, and it is possible that by means of such 

 mycorhizae, especially when endophytic, the older roots may remain 

 capable of absorbing water and dissolved substances. 



The character of a root-system is the result of the interaction 

 between its inherent specific peculiarities and the external conditions under 

 which its development takes place. It is owing to the latter that the root 

 or rhizomes of the same plant penetrate more deeply in loose soil than 

 in tough clayey loam, and that in its lateral development the root-system 

 does not maintain the same distance from the surface at all points. This 

 distance is mainly influenced by the aeration and distribution of water, but 

 temperature and illumination are also of importance. An exact determi- 

 nation of the precise influence of each different factor is extremely difficult, 

 for abundant aeration retards nitrification, while other agencies, including 

 putrefactive changes, may alter the conditions existing in the soil. 



The shape and position of the root-system must obviously be adapted 

 as far as possible to the external conditions, and thus in cereals the main 

 development of the root-system occurs in the same layers of the soil, 

 whether the seeds were sown near the surface or buried deep in the ground 4 

 (Fig. 14), while when a rhizome is planted at some distance below the 

 surface, the new increments are directed upwards until the normal depth 



1 Observed long ago by linger, Anatomic, 1855, P- 39- F r details see Fr. Schwarz, Unters. 

 a. d. Bot. Inst. z. Tubingen, 1883, Bd. I, p. 135, where the literature is given. 



a Nobbe, Versuchsst., 1862, Bd. IV, p. 217, and 1868, Bd. X, p. 94; Pfeffer, Druck und Arbeits- 

 leistung, 1893. 



3 See Fr. Schwarz, I.e., p. 148. Cf. also Went, Bot. Centralbl., 1894, Bd. LIX, p. 367. [In 

 the aerial roots of Vanilla aromatica, moisture is essential for the formation of the root-hairs ; 

 darkness and contact accelerate, while light and dryness retard it : Ewart, Contact Irritability, Ann. 

 d. Jard. bot. de Buitenzorg, 1898, p. 236.] 



* Literature : G. Kraus, Forsch. a. d. Geb. der Agr.-Physik, 1889, Bd. xn, p. 259; 1894, Bd. 

 XVII, p. 35 ; 1896, Bd. xix, p. 17 ; Kossovvitsch, ibid., 1894, Bd. xvn, p. 104. 



