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On the Origin and Formation of Roots. In a Letter from Thomas 

 Andrew Knight, Esq. F.R.S. to the Right Hon. Sir Joseph Banks, 

 K.B.P.R.S. Read February 23, 1809. [Phil. Trans. 1809, ;>.1G9.] 



The object of this paper is to show, that the roots of trees are al- 

 ways generated by the vessels which pass from the cotyledons of the 

 seeds, or from the leaves through the leaf-stalks and bark, and that 

 they never spring immediately from the alburnum. 



The radicle, which proceeds from the seed, appears to the author 

 to differ from other roots in its mode of growth, since it elongates, 

 by interstitial increase, like the intervals between the buds in the 

 succulent annual shoot ; but roots, on the contrary, elongate only by 

 new parts added to their extremity, and never by the extension of 

 parts previously formed. 



The proper roots, which come first into existence, spring from the 

 point of the radicle ; and since there is at that time no alburnum, it 

 is evident they must arise from some other source. 



At first they consist solely of cellular substance, within which cor- 

 tical vessels are next generated ; by these the alburnum is subse- 

 quently deposited, in the form of wedges, meeting in the centre. 

 .. If a portion of bark be removed from a vine, in a circle, round the 

 stem, and any wet substance be applied to it, roots are soon emitted 

 from the upper edge of the decorticated space ; and when the al- 

 burnum dies so as to obstruct the progress of sap through it, buds 

 are usually protruded from the lower edge, but never from the upper; 

 the roots deriving their matter from the fluid that descends through 

 the cortical vessels, and buds from the ascending sap. 



In some varieties of the apple-tree, Mr. Knight observes, there are 

 many rough excrescences on the trunks and branches, which, under 

 different circumstances, form either buds or roots, and these varieties 

 are accordingly very easily propagated by cuttings. When such ex- 

 crescences had begun to form upon some trees of two years old, mould 

 was applied to some of them in the spring, and roots were found to 

 form early in the summer. But when mould was applied to other 

 trees of the same age and variety, from which the top had been cut 

 at a short distance above the excrescence, no roots were emitted for 

 want of descending sap, but buds were formed instead. 



The author observes, that both alburnum and bark contain true 

 sap ; but whether that which descends to form roots differs essentially 

 from that which ascends to form buds, he thinks it nearly impossible 

 to decide : he is, however, much more disposed to attribute the for- 

 mation of different organs to the different action of the vessels, than 

 to any difference of the fluids from which they are formed. 



After alburnum has been formed in the roots, it then has the power 

 of producing buds from its upper extremity, as well as fibrous roots 

 from its lower extremity. The continuance of the entire root in the 

 state of alburnum, appears owing to moisture ; for if the mould be 

 taken away so as to expose part of a root to the air, that part is sub- 

 sequently found to contain heart wood. 



The formation of buds from the potatoe, beneath the soil, may ap- 



