VIII.] VASCULAR BUNDLES. I15 



12. Besides the distinction which I have pointed out 

 between cells and vessels, you may roughly group the 

 different kinds of cells as lo7ig or short. Long cells are 

 usually thick-sided, and often taper at each end, so that 

 when a number of them have grown together, forming a 

 tissue, we find such tissue to be generally firm and tough. 

 Such cells, together with a few vessels, form the principal 

 mass of wood, of petioles, and of the veins of leaves. These 

 veins, which have nothing in common with the veins of 

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

 short cells which occupy their interstices. The short cells 

 of leaves are generally thin-walled, and during spring and 

 autumn they are 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 the 

 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. 



13. 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 difference in 

 the- mode of arrangement of their fibro-vascular bundles 

 becomes apparent. And this difference essentially consists 

 in the circumstance that in Monocotyledons the fibro- 

 vascular bundles remain permanently isolated, and, once 

 completed in the stem, do not receive any addition in thick- 

 ness, while in Dicotyledons they become confluent, forming 



