LECT. V.] THE ROOT. 201 



root, Mr. Knight has advanced sufficient reasons 

 for believing that this organ is not an elongation 

 of the radicle; but is formed upon it after the 

 germination of the seed by successive increments 

 of new matter deposited at its apex ; for, whilst 

 the root never elongates by the extension of al- 

 ready organized parts, the radicle does elongate 

 throughout all its parts in the germinating seed. 

 The root, at first, consists of cellular matter only, 

 contained in an epidermis, within which the cor- 

 tical vessels are afterwards generated ; the circle 

 of vessels, says Mr. Knight, " enclosing within it 

 a small portion of the cellular substance, which 

 forms the pith, or medulla of the root ; and these 

 vessels gradually generate alburnum, which, in a 

 transverse section of the root, appears arranged in 

 wedges round the medulla." The same able phy- 

 tologist further maintains that root shoots differ 

 from stem shoots in not being emitted from the 

 alburnum ; and that the presence of the alburnum 

 is not essential is evident from the fact, that leaf- 

 stalks, which contain no alburnum, nevertheless 

 emit roots ; and these derive their existence, as in 

 all other cases, from the proper juice conveyed in 

 the returning vessels *. 



All roots have originally more or less of a 

 tapering form ; which is partly the consequence 



* Phil. Transactions, 1809. 



