86 



BOTANY OF THE LIVING PLANT 



the Buttercup, which has five protoxylem groups, a number not 

 uncommon for Dicotyledons. But of these only the protoxylem 

 vessels are 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 seedlings in various 

 ways, at or near to the level of the soil. The xylem-masses rotate upon their 

 axes, and this is combined with splittings and fusions of the strands 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 characteristic structure of the root passes upwards into that of the 

 stem. 



In order that the root may absorb water from the soil, a close relation 

 with the soil must be established. This is effected by the root-hair. 



Fig. 60. 



Representation of root-hairs in the soil. £ = piliferous layer of the root. h, h'= 

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

 of the soil. Each of these is covered by a film of water, which is shaded ; while the 

 clear spaces indicate the air-cavities in the porous soil. (After Sachs.) 



The parent cell 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 



