viii.] CELLULAR AND VASCULAR SYSTEMS. 99 



veins of animals, serve as a sort of framework for the 

 support of the short cells, which occupy the/ir interstices. 

 The short cells of leaves are generally thin-walled, and 

 during summer their chlorophyll is busily engaged in 

 elaborating the food of the plant by the aid of the sun's 

 light and heat. The bundles of long, thick- walled cells 

 with the vessels which accompany them, forming the 

 veins, we may speak of as \\\Q fibro-vascular system, and 

 the short cells as the cellular system of the leaf. In 

 the petiole the cellular system is much reduced, and the 

 fibro-vascular system is contracted into narrow compass. 



ii. The arrangement of these systems, as they are 

 termed, in the stem, differs considerably in the two great 

 Classes of flowering plants. 



Excepting in their single cotyledon and the behaviour 

 of the radicle in germination, Monocotyledons are not, at 

 first, materially different from Dicotyledons ; but when 

 one or two seasons of growth are over, a marked differ- 

 ence in the mode of arrangement of their fibre-vascular 

 bundles usually becomes apparent. And this difference 

 essentially consists in the circumstance that in Mono- 

 cotyledons the fibro-vascular bundles of the stem remain 

 permanently isolated, and once completed in the stem do 

 not receive any addition in thickness ; while in Dicotyle- 

 dons they become confluent, forming a continuous ring 

 round the pith, and constantly increase in thickness 

 during the successive working seasons of the tree by 

 organically continuous additions to their outer side : so 

 that in Monocotyledons the bundles are closed or definite j 

 in Dicotyledons, continuous or indefinite. 



But the nature of this difference you will appreciate 

 better when you understand the composition or arrange- 

 ment of the tissues forming these fibro-vascular bundles. 

 Each bundle includes at first a layer of cells of extreme 

 delicacy, which cells are capable of undergoing division 

 and enlargement, and it is by means of this layer only 

 that the bundle can increase in thickness. This layer of 

 active cells is inclosed between two distinct but yet 

 corresponding systems : one system (the xylem system) 

 usually on the side towards the centre of the stem, in our 

 trees of long, thick-walled cells and vessels, forming what 

 is popularly understood as the wooa of the fibro-vascular 



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