STRUCTURE OF THE ROOT 



75 



uncuinmon lor Dicotyledons. But ol these only the protoxylem 

 vessels are as yet developed ; tlie 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 iirrangenient 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 

 different 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. 



F}G. 57- 



1-ieprtsc-ntatiou 6i roat-liairs iu the soil. £ = epidermis of a vertical root. /(, /j' = 



root-hairs gro-wn out from its cells, and adjusting their growth to the solid fragments 



of the soil. Each of these fragments is covered by a Irlm of water, which is shaded ; 



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



This is the important duty 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 



