32 Oil the Origin and Formation of Roots. 



vegetate. The mode of growth of the radicle is therefore 

 siniilar to that of the substance which occupies the spacel 

 between t"he buds near the point of the succulent annual 

 shoot, and totally different from that of the proper root of 

 the plant, which I conceive to come first into existence du- 

 rins: the germination of the seed, and to spring from the 

 point of what is called the radicle. At this period, neither 

 the radicle nor cotyledons contain any alburnum ; and there- 

 fore the first root cannot originate from that substance ; but 

 the cortical vessels are then filled with sap, and apparently 

 in full action, and through these the sap appears to descend 

 which oivcs existence to the true root. 



When first emitted, the root consists only of cellular sub- 

 stance, similar to that of the bark of other parts of the fu- 

 ture tree, and within this the cortical vessels are subsequently 

 generated in a circle, inclosing within it a small portion of 

 the cellular substance, which forms the pith or medulla of 

 the root. The cortical vessels soon enter on their office of 

 generating alburnous matter; and a transverse section of the 

 root then shows the alburnum arranged in the form of 

 wedges round the medulla, as it is subsequently deposited 

 on the central vessels of the succulent annual shoot, and on 

 the surface of the alburnum of the stems and branches of 

 older trees*. 



If a leaf-stalk be deeply wounded, a cellular substance, 

 similar to that of the bark and young root, is protruded from 

 the upper lip of the wound, but never from the lower; and 

 the leaf-stalks of many plants possess the power of emitting 

 roots, which power cannot have resided in alburnum, for 

 the leaf-stalk does not contain any ; but vessels, similar to 

 those of the bark and radicle, abound in it, apparently con- 

 vey the returning sap; and from these vessels, or perhaps 

 more properly from the f^uid they convey, the roots emitted 

 by the leaf-stalk derive their existence f. 



If a portion of the bark . of a vine, or other tree, which 

 readily emits roots, be taken off in a circle extending round 

 its stem, so as to intercept entirely the passage of any fluid 



• Philosophical Transactions fo» 1801, Plate nvii. tit'''^-^or 1801, 



through 



