208 VEGETABLE ORGANOGRAPHY. 



enlarge ahnost indefinitely. It appears to me clear that 

 this difference is owing only to the degree of solidity or 

 hardness which the tissue of each species acquii'es. 

 When the old fibres, pushed towards the outside by 

 the interposition of young ones at the centre, are har- 

 dened at a certain age, they serve as a solid sheath to 

 all the central bundle, and the stem does not increase in 

 diameter; this is what takes place in Palms. "When 

 these same fibres always preserve their flexibility or sup- 

 pleness, so as to be able to be more or less distended by 

 the interposition of the central fibres, the stem can always 

 increase in diameter ; it is this which happens in herba- 

 ceous Liliacege, and almost all Endogens with soft tissue. 



CHAPTER II. 



OF THE ROOTS OF VASCULAR PLANTS. 



Section I. 

 Comparison of Stems and Roots. 



It is usual to call the Root, in ordinary language, the 

 part of plants which is buried under ground ; and a 

 celebrated botanist (Hedwig) wished to found upon the 

 popular character the same definition of the root, which 

 he considers as differing from the trunk only in its 

 position, and he calls it Truncus suhterraneus. But 

 this definition is not correct ; the stems of Ferns and 

 Liliaceoe are sometimes aerial, at others subterranean ; 



