OF THE ALBURNUM OF TREES. 153 



and acquires many degrees of increased specific gravity ; and a similar 

 absorption of air, with corresponding effects, is well known to take place 

 in the process of malting. 



I shall conclude with observing, that in retracting the opinion I for- 

 merly entertained respecting the ascent of the sap in the alburnous tubes, 

 I do not mean to retract any opinion that I have given in former commu- 

 nications respecting the subsequent motion of the sap through the central 

 vessels, the leaves, and bark ; or the subsequent junction of the descend- 

 ing with the ascending current in the alburnum : every experiment that 

 I have made has, on the contrary, tended to confirm my former con- 

 clusions. 



XII. ON THE ORIGIN AND FORMATION OF ROOTS. 

 [Read before the ROYAL SOCIETY, February, 23, 1809.] 



IN a former communication I have given an account of some experi- 

 ments, which induced me to conclude that the buds of trees invariably 

 spring from their alburnum, to which they are always connected by 

 central vessels of greater or less length ; and in the course of much sub- 

 sequent experience, I have not found any reason to change the opinion 

 that I have there given*. The object of the present communication is 

 to show, that the roots of trees are always generated by the vessels 

 which pass from the cotyledons of the seed, and from the leaves, through 

 the leaf-stalks and the bark, -and that they never, under any circum- 

 stances, spring immediately from the alburnum. 



The organ which naturalists have called the radicle in the seed, is 

 generally supposed to be analogous to the root of the plant, and to 

 become a perfect root during germination ; and 1 do not know that this 

 opinion has ever been controverted, though I believe that, when closely 

 investigated, it will prove to be founded in error. 



A root, in all cases with which I am acquainted, elongates only by new 

 parts which are successively added to its apex or point, and never, like 

 the stem or branch, by the extension of parts previously organised ; and 

 I have endeavoured to show, in a former memoir, that owing to this dif- 

 ference in the mode of the growth of the root and lengthened plumule of 

 germinating seeds, the one must ever be obedient to gravitation, and 

 point towards the centre of the earth, whilst the other must take the 

 opposite direction t. But the radicle of germinating seeds elongates by 



* Sec above, No. VI. f Ibid. No. VII. 



