THE ANGIOSPERMAE : ROOTS 799 



with a denser aggregation near the apex, in which the nucleus usually lies. 

 The full development only takes a few hours and the length reached may 

 vary between 0-2 and 5 mm., or in damp air even more. In the soil the 

 primitive form is greatly distorted by contact with soil particles, to w^hich 

 the wall of the hair adheres firmly and the hair may even become branched. 



The growth of these hairs seems to be correlated with the stoppage of 

 expansion in the cells which bear them. It is to be observed that they appear 

 just above the principal elongating zone of the root, that is to say, their 

 appearance coincides with the cessation of cell elongation. Furthermore, if 

 roots of the same species growing under different conditions are compared, 

 a greater production of hairs is associated with shorter basal cells, w^hile the 

 trichoblasts, in those species where they are specially formed, are always 

 much shorter than the other surface cells. The appearance of the root hairs 

 behind the zone of cell elongation prevents their being broken off by the 

 forward growth of the root. 



The growth of the hair is preceded by a change in the middle lamella 

 and in part of the outer wall of the basal cell, from pectic acid to Calcium 

 pectate. The tip of the young hair is formed from a portion of wall which 

 contains less Calcium than the rest and is in consequence softer and more 

 delicate. The hair grows entirely by the stretching of this soft tip, and the 

 fresh substance added to it from the protoplasm is hardened by addition of 

 more Calcium on the sides of the hair. Oxalic acid, which hinders pectate 

 formation, or an excess of Calcium, which has the opposite effect of causing 

 complete pectate formation all over the wall, both prevent the formation of 

 hairs. Some older hairs, especially in Gleditschia and Eupaton'um, show 

 some lignification and may persist even on the older roots. In the Com- 

 melinaceae persistent root hairs develop from the exodermis after the 

 normal root hairs have disappeared. 



The life of a root hair normally ends as rapidly as it begins. Shortly 

 after reaching full length the hair dies and drops off, taking w'ith it the surface 

 cell from which it grew. In this way the whole of the piliferous layer dis- 

 appears, leaving the hypodermal layer, called the exodermis, as the new 

 surface of the root. The cell walls of this layer are often impregnated with 

 fatty substances. 



The growth of root hairs obviously increases the total surface of the root, 

 which may be important from the point of view of absorption. Actual 

 measurement shows that this increase of surface is exaggerated by the eye. 

 It may be as much as ten times, but it is more often of the order of three to 

 five times. Some of the higher ratios are found among aquatics, and this 

 raises the question why it should be so, as they apparently need the increased 

 absorptive capacitv less than do land plants. It may be pointed out that 

 although root hairs increase the absorptive surface in contact with the soil, 

 they do not affect the absorptive capacity of the root to a corresponding 

 extent, since all materials absorbed by them must pass into the root itself 

 through the base of the hair. Thus the effective assimilating surface of the 

 root is unchanged by the development of the hairs. Priestley has suggested 



^ 



