STRUCTURE OF THE ROOT 



77 



B.C. 



The hairs arise in acropetal order. Individually they are functional 

 only for a short time, as is seen from the fact that though active at 

 a short distance backwards from the growing root-tip, at a further 

 distance from the apex they may have already shrivelled away, 

 so that the older part of the root no longer preserves its intimate 

 relation to the soil. Thus as the root-system extends, it taps an 

 ever larger area of the soil, while it is actively absorbent only at 

 the outer limit of the area invaded (Fig. 58). 



As soon as any part of the root is anchored in the soil by its out- 

 growing hairs, it could not possibly increase in length without tearing 

 them away from their hold. But 

 of this there is no sign. It fol- 

 lows, therefore, that the 'growth 

 in length of the root must be 

 restricted to the region beyond 

 the youngest root-hairs. It may 

 be demonstrated that the growth 

 is thus restricted, and that the 

 most rapid growth is close to 

 the tip, by making marks with 

 Indian ink at equal distances 

 upon the outside of a growing 

 root. The root should then be 

 kept in condition as near as may 

 be to the normal. After a period section > and placed apex upwards. u.c=root 



f cap. cai=calyptrogen laye,r, which renews the 



Of twenty-four hours, if the dis- tissue of the cap. C/=coWion initials for pili- 



J ferous layer (Pt7)and the periblem (Peri). Pler= 



tanCC between the marks be central cylinder of plerome, giving rise to the stele 



. . (xiio.) (After Janczewski.) 



compared with the original scale, 



it will be seen that the two do not tally. It will easily be seen where 

 the greatest elongation has been (Fig. 59)- The most rapid growth 

 in an average root is about 5 mm. from the tip. It diminishes 

 gradually from that point in both directions, and ceases at about 

 IO mms. from the tip. This restriction of the growth in length to a 

 short region behind the tip is characteristic of roots, and is in 

 sharp contrast to what is seen in ordinary stems. In them the 

 growth in length may be spread sometimes over a length of several 

 decimetres. 



A consequence of the growth of the region beyond the anchoring 

 root-hairs is that the root-tip itself is forced forward. As in the stem, 

 so in the root the apical tissue is embryonic, and of delicate texture. 

 Nevertheless this delicate root-tip is driven through the soil, forcing 



FIG. 60. 

 Root-tip of Barley, cut in median longitudinal 



