222 



ABSORBING SYSTEM 



The great elongation of a root-hair in moist air, after all, only represents 

 a means to an end ; it is a response, which enables the hair to traverse 

 considerable interstices and cavities in the soil, and so ultimately to 

 come into contact with fresh solid particles. Absence of light, as is well 

 known, causes an excessive elongation of aerial organs ; consequently, 

 even if a shoot begins its development far below the surface of the 

 ground, it is usually able to reach the light sooner or later. Similarly, 

 absence of contact with solid bodies causes an excessive elongation of 

 root-hairs, whereby these structures are ultimately enabled to penetrate 



into solid ground. Evidently these 

 two phenomena, though so widely 

 different in their physiological sig- 

 nificance, are nevertheless closely com- 

 parable from an ecological point of 

 view. The above-mentioned modifi- 

 cations of the root-hairs must un- 

 doubtedly be regarded, not as cases of 

 mere arrested development, but rather 

 as responses to the stimulus of contact 

 with solid particles. 



The connection between root-hairs 

 and the soil-particles to which they 

 adhere, is usually rendered more 

 intimate and permanent by a pro- 

 cess of agglutination : the existence of this process was assumed by 

 Charles Darwin, and the phenomenon has since been more closely 

 studied by Schwarz, who attributes it to gelatinisation of the outermost 

 layers of the root-hair membrane. 



The type of longitudinal growth exhibited by root-hairs entirely 

 accords with the manner in which they penetrate the soil and glue 

 themselves to solid particles. According to observations carried out by 

 the author 118 upon root-hairs of Cucurbita Pepo, Pimm sativum, Poly- 

 gonum Fagopyrum and Helianthus minims, which had been marked by 

 dusting them with rice-powder, longitudinal growth is strictly confined 

 to the dome-shaped tip of the hair, and ceases altogether immediately 

 behind the apex. Boot-hairs thus exemplify an extreme type of 

 apical growth, and in this respect also show themselves to be 

 exceedingly well fitted for the task of pushing their way between the 

 solid particles of the soil. The significance of the very close apposition 

 of the root-hairs to solid particles only becomes fully evident, when one 

 considers how the water and nutrient salts are distributed within the 

 soil. As a matter of fact only a small part of the most important 

 food-constituents exist in a state of solution in the soil-water ; by far 



Fig. 84. 

 Root-hairs of Linaria Cvmbalaria. 



