104 



BOTANY 



PART I 



the xylem there is a succession of annular, spiral, reticulate, and finally 

 pitted vessels (Fig. 120 B). The first-formed elements of xylem and 

 phloem have ceased to be functional in the fully-developed vascular 

 bundle. The protoxylem elements are then frequently compressed 

 or torn by the stretching (Fig. 120 /, at a, a), and in some cases their 

 place is taken by a lysigenous intercellular space (Fig. 120 I). The 



FIG. 121. Transverse section of an open collateral vascular bundle from a stolon of Ranunculus 

 repens. s, Spiral tracheides ; m, vessel with bordered pits ; c, cambium ; v, sieve-tubes ; vg, 

 sheath. ( x 180. After STKASBURGER.) 



walls of the protophloem elements (cp) are swollen and their sieve- 

 plates closed by callus. 



In radial bundles the differentiation proceeds, in accordance with 

 the position of the first formed elements in the strands of xylem and 

 phloem, from the periphery towards the centre. In collateral bundles, 

 on the other hand, the elements are developed in succession from the 

 protophloem on the outside and the protoxylem on the inside towards 



