VIII.] EXOGENS. \\1 



Dicotyledons all the wood is on the inside of this cambium- 

 cylinder, and new wood is deposited on the outside of wood 

 previously formed ; all the liber, 

 on the other hand, is on the out- 

 side of the cambium, and im- 

 mediately within the bark — of 

 which, indeed, the liber is re- 

 garded as forming an inner layer. , 

 Structure such as here described \ ^fe^^^ji^^K^ 

 is precisely what we find in the \ ^^^tijy^~^ / 

 stem of a Mango, or any other ^^"^-^Jl^--^^ 



dicOt} ledonOUS tree, which is said ^^,^_ ^. Diagram showing the ar- 



to be exogenous, from the circum- ^uS^'^Lch 5^.^'^:^ 

 stance that the wood increases '^^ti.^..::^'^:Sy^^:i^. 

 by additions to its outside. X^i:t:rS'^$^.^^ tt 



In Monocotyledons, on the cartes Uiefuture cam omm-cylmder; 



Other hand, the cambium-cells 



of the different fibro-vascular bundles never coalesce so 

 as to form a cambium-cylinder : consequently they do not 

 form continuous rings of wood. The cambium-cells, there- 

 fore, soon cease their dividing and enlarging work, and the 

 Tibro-vascular bundle is finished. We find, if we cut the 

 solid stem of a Monocotyledon across, that the tibro -vascular 

 bundles are irregularly scattered all through the cellular 

 system of the stem. They are especially crowded towards 

 the circumference, which consequently becomes much 

 harder than the centre in Woody Monocotyledons. From 

 the mode of development of the fibro-vascular bundles, and 

 the direction which they take in the stem, the trunk of 

 Woody Monocotyledons does not usually increase in diameter 

 beyond a certain point, as we find in Palm-trees, which 

 frequently have tall cylindrical stems as thick at the top as 

 at the base. These peculiarities led the older botanists 



