STRUCTURE OF THE ROOT 



75 



uncommon for Dicotyledons. But of these only the protoxylem 

 vessels arc as yet developed ; the vessels of the metaxylem are still 

 thin-walled, but they extend to the centre of the pithless root, and 

 they form a solid star of xylem when mature. 



Since the arrangement of the vascular tissues is radial in the root, but 

 collateral in the stem, it is obvious that a readjustment must take place where 

 the one passes into the other. The change is effected in various ways in 

 difierent plants, at or near to the level of the soil. The xylem- masses rotate 

 upon their axes, and this is combined with splittings and fusions in some cases, 

 so that the peripheral protoxylem of the root becomes central in the stem, 

 and the xylem-masses range themselves internally to the phloem-masses. 

 Thus without break of the continuity of the conducting tracts, the charac- 

 teristic structure of the root passes upwards into that of the stem. 



In order that the root may perform its function of absorption of 

 water from the soil, a close relation with the soil must be established. 



Fig. 57- 

 Ropresentation of root-hairs in the soil, c^epideriiiis of a vertical root. //, h' = 

 root-hairs grown out from its colls, and adjusting their growth to the solid fragments 

 of the soil. Each of these fragments is covered ty a film of water, which is shaded ; 

 while the clear spaces indicate the air-cavities in the porous soil, (.\fter Sachs.) 



This is the important duly of the root- hair. The parent cell that gives 

 rise to it is usually oblong in form, and from a point about the middle 

 or upper end of its outer face the hair arises as a cylindrical process, 

 which penetrates between the particles of the soil. It adjusts its 

 form to the spaces between them, while the nucleus passes out into 



