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MANUAL OF BOTANY 



lie towards the centre of the cylinder, and later others appear 

 nearer the periphery. Further development leads to increasing 

 irregularity in the ring-like arrangement, so that in an older 

 stem the bundles seem to be scattered irregularly in the cylinder 



(fig- 751). 



The bundles are of the collateral type, but as differentiation 



Fig. 752. 



Fki 752. Collateral bundle from stem of a Monocotyledon, ph. Phloem. 

 X Xylem vessels, p.ph. Protophloijm. p.x. Protoxylem. The bundle is 

 surrounded by a small-celled sheath of solerenehyma. After Kny. 



of the tissues proceeds, all the procambium is transformed into 

 permanent tissue, so that the bundles are closed, containing no 

 cambium. The protoxylem is at the apex of the bundle, and 

 consists of one or two spiral or annular vessels {fig. 152, p.x). 

 The large ducts or vessels on the wings of the bundle are next 

 developed and then the remainder of the wood. The proto- 

 phloem is at the back of the bast (fig. 15% p.ph). The whole 



