THE STRUCTURE OF THE ROOT 379 



latter, the protoxylem as well as the protophloem is situated on 

 the outer face of the bundle, so that the development is always 

 centripetal. This constitutes a further important distinction 

 between the root and the stem. 



The stele of the root does not usually contain any pith, the 

 xylem bundles becoming united towards the centre. When 

 pith is present the walls of its cells are often liguified, so that 

 the centre of the stele is a hard woody cylinder. 



The number of the xylem and phloem bundles varies, being 

 usually much greater in Monocotyledons than in Dicotyledons. 

 Generally there are as many of one as of the other. When only 

 two of each are present the mass of wood formed by the union 

 of the xylems extends across the cylinder, forming a plate of 

 tissue, and the two bundles of the phloem lie one on each side of it. 

 Such a root is called diarch. When more xylem bundles unite 

 to form the plate, corresponding terms are used to describe it ; 

 thus we have triarch, tetrarch, polyarch roots. These terms 

 were originally used when it was considered that the central mass 

 of wood was a single bundle, with several points of origination of 

 the differentiation. 



The roots of Dicotyledons and Gymnosperms do not contain 

 any meristem homologous with that in the bundles of the stem. 

 When they increase in thickness, they do so by the development 

 of interfascicular cambium, which arises as a secondary meristtm 

 in the conjunctive tissue on the inner face of the phloem bundles, 

 and which forms wood on its inner face and bast on its outer 

 as in the stem. These interrupted strands of cambium are a 

 little later connected together by a similar meristem arising in 

 the pericycle. The cells of the latter divide so as to form two 

 or more layers in thickness behind the primary xylem bundles, 

 and the inner layer becomes merismatic. In this way a sinuous 

 layer of cambium is formed, which becomes circular as growth 

 proceeds. The further development is similar to that of the stem. 



The behaviour of the cambium ring is not uniform in differ- 

 ent plants. In some it forms wood and bast opposite only to 

 the original phloem bundles, giving rise to parenchyma behind 

 the primary xylems [fig. l&l). Thus a broad medullary ray 

 alternates with strands of secondary vascular tissue, as in Urtica, 

 Cucurbita, and many other plants. This is very prominent 

 generally in fleshy roots. Frequently in the latter, in the width 

 of this ray, separate intermediate strands of vascular tissue occur. 

 In other cases the cambium forms bast and wood opposite to 

 both primary phloem and primary xylem. 



