XXXIV Venation 463 



is separated from the cortical parenchyma {corf) by a la\er 

 of sclerenchymatous cells, the pericyde {pre). The bundles 

 are collateral, the xylem {xy) facing the axis of the stem, 

 the phloem {phi) its periphery : but there is a fundamental 

 difference from the bundles of Gymnosperms and Dico- 

 tyledons in that the fully formed bundle contains no 

 cambium, and is therefore incapable of further growth. The 

 bundles of Monocotyledons are therefore closed, while those 

 of Gymnosperms and Dicotyledons are open. Owing 

 partly to this circumstance, partly to the thick unyielding 

 pericycle, the stems of nearly all Monocotyledons are in- 

 capable, when once their tissues are fully formed, of further 

 increase in thickness. Hence the characteristic slenderness 

 of the trunk of a palm as compared with that of a pine or 

 an oak. 



The wood of Angiosperms consists of spiral, annular 

 and dotted vessels, of fibres or prosenchymatous cells, and 

 of parenchyma. The phloem contains sieve-tubes, long 

 tough prosenchymatous cells called bast-fibres, and paren- 

 chyma. The growing point, as in Gymnosperms, has no 

 apical cell. 



The leaves vary indefinitely in form, and all that can be 

 mentioned with regard to them in the present brief sketch 

 is that in most Monocotyledons they are long and narrow, 

 and traversed by numerous parallel veins, while in Dico- 

 tyledons they are generally broad, with a smaller number — 

 one to five — of primary veins from which secondary veins 

 branch out and unite in a network. So that the venation 

 or veining is parallel in Monocotyledons, reticulate in Dico- 

 tyledons. 



It is in the structure of the flower that the most striking 

 differences from, and the most marked advance upon, 



