1902] EV0LU770N OF VASCULAR TISSUE OF PLANTS '2.2 ^ 



portion as the tissue in this region increased in thickness, there 

 took place, correlative!}^ therewith (what, indeed, would be, a 

 priori, obviously expected) a gradual reduction of the centripetal 

 vascular tissue, viz., that on the medullary side of the protoxylem, 

 whether secondary or primary. This can be seen beautifully 

 exhibited within the Medulloseae group itself, eventually culmi- 

 nating in the stem structure of modern cycads. The process 

 of elimination of the centripetal xylem proceeded apace, until 

 finally the endarch stem structure of Cycads, Coniferae, Cordaites, 

 Gnetaceae, and Dicotyledons, as being by far the most econom- 

 ical and advantageous method of vascular formation, eventually 

 prevailed. 



In the case .of leaves there has not been such a demand for 

 modification of the vascular tissues as was the case with the 

 stem, hence the longer persistence of the older character in 

 these organs. 



In roots the primitive centripetal structure of the wood has 

 been far more persistent, probably for the purpose of enabling 

 these organs to resist central tension strains. 



In Monocotyledons, although true secondary thickening does 

 not occur, the endarch type of bundle nevertheless obtains ; 

 where the concentric structure is found, as in those cases of 

 bundles with a central phloem, this is merely to be regarded as 

 a modification of the endarch t3^pe. 



Kevv, Surrey, England, 



SI 



