142 ANATOMY OF STEM, ROOT AND LEAF 



side on separate radii drawn from the centre of the root with 

 small intervening bands of ground-tissue between them. 



Moreover, in roots the first-formed, narrow-bored elements (x) 

 of the primary wood are nearest the outside, while in stems they 

 are nearest the centre. 



According as the number of separate strands of wood is two, 

 three, or many, the roots are described as diarch, triarch (as in 

 Fig. 72), ox poly arch respectively. 



The number of rows of secondary roots generally corresponds 

 to the number of strands of primary wood in the parent root, 

 each row being formed in the pericycle almost opposite a wood 

 strand. 



In all roots the development of the primary wood proceeds in- 

 wards and frequently it goes on until the several strands unite to 

 form a mass which occupies the centre to the complete exclusion 

 of pith. Nevertheless, in some roots, and especially those of 

 monocotyledonous plants, pith is present. 



The roots of perennial dicotyledons increase in thickness 

 just as the stems do, but owing to the different disposition of 

 the primary tissues the first formation of the cambium is not 

 the same as in a stem. In roots the cambium first forms in 

 the ground-tissue on the inside of the bast-strands, and sub- 

 sequently within the pericycle opposite the primary wood ; in 

 transverse sections, therefore, the cambium in the early stages of 

 its existence appears as a wavy band of meristem (2, c. Fig. 73). 



When active growth of the cambium takes place, the wavy 

 outline is soon lost and it is then seen as a simple ring of 

 meristem, producing secondary wood and bast in a manner 

 precisely similar to the cambium of an ordinary stem. 



In roots which grow in thickness, a phellogen arises in the 

 pericycle and like that of thickening stems produces cork ex- 

 ternally and phelloderm internally. In consequence of the 

 formation of a ring of cork by the phellogen, all the tissues 

 external to it, namely, the endodermis, primary cortex and 



