384 MANUAL OP BOTANY 



divides into two, and the separate halves diverge to the right and 

 left, twisting as they proceed, till the protoxylem, at first on the 

 outer face, becomes internal. The phloem bmrdles also bifurcate 

 and diverge as do the xylem ones, passing gradually a little 

 towards the exterior. Soon the left strand of a phloem bundle is 

 found to be exactly behind the right strand of the adjacent 

 xylem bundle, the two forming thus an ordinary collateral bundle 

 which passes perpendicularly up the stem. The xylem and 

 phloem bundles of the stem are thus twice as numerous as those 

 of the root, though by the fusion of the separate strands of bast 

 and wood the number of conjoint collateral bundles of the former 

 corresponds with the nunrber of separate bundles of the latter. 

 The phloem strands do not twist in their course, so that the 

 protophloem remains external. 



Less frequently the phloem bundles pass straight wp into the 

 stem without branching. The xylem bundles branch, diverge 

 and twist as in the former case. The right and left halves of 

 two contiguous xylem bundles meet in front of the continuation 

 upwards of the phloem bundle originally between them, fuse, 

 and become the xylem portion of the conjoint bundle. 



In a third case the xylem bundles do not branch, but as they 

 pass upwards twist as before through an angle of 180^. The 

 phloem bundles branch and diverge, the halves of two contiguous 

 ones fusing together behind a xylem bundle and uniting with it 

 to form the conjoint bundle. 



In some plants, particularly among the Bosacece, the roots 

 give rise to adventitious buds. Instead of these being exogenous, 

 as when they spring from the stem, they originate deep in the 

 tissue. They are derived from the pericycle like the lateral 

 roots, and bore their way out in the same way as the latter. 

 Linaria offers an exception to this mode of origin, as its radical 

 biids arise from the epidermis. 



