ON THE ORIGIN AND roRMATION OF ROOTS. 1]Q 



tension of parts previously orj^anized, and in a great num- seeds elonga- 



ber of cases, which must he familiar to every person's oh- ^'. by exion- 



*' "^ sioii or oiga- 



servation, raises the cotyledons out of the mould in which ni/ed parts. 



the seed is placed to vegetate. The mode of growth of the 

 radicle is therefore similar to that of the substance which 

 occupies the spaces between the 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 Origin of the 

 inter existence during the gemination 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 albur- 

 num ; and therefore the first root cannot originate from that 

 substance; but the cortical vessels are then tilled vvilh sap, 

 and apparently in full action, and through these the sap ap- 

 pears to descend, which gives existence to the true root. 



When first emitted, the root consists only of a cellular Its form»ti«n. 

 substance, similar to that of the bark of other parts oi the 

 future tree; and within this the cortical vessels are subse- 

 quently generated in a circle, enclosing 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 suri'ace of the alburnum of the stems and / 

 branches of older trees*. 



If a leaf-stalk be deeply wounded, a cellular substance, r.eaf-stalks oi 



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



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 tliough they 



of the bark and radicle, abound in it, and apparently convey couiain uoal- 



1^1 1 , biirnum, 



the returning sap; and irom these vessels, or perhaps more 



properly from the fluid they convey, the roots emitted by 



the leaf-etalk derive their existencef . 



* Phil. Trans, for 1801, Plate 27. 

 fPhil.Trans. for 1801. 



