1855.] Linnean Society. 365 



He thinks that the formation of a circular tumour in the trunk of 

 dicotyledonous plants above the line of a ligature tightly tied around 

 it may be accounted for by reasoning on the facts which he con- 

 ceives himself to have established, viz. that in the development of 

 the vascular fibres of the stem, there always exists a vital centre from 

 which they extend themselves in two opposite directions. This 

 vital centre may be fixed, moveable, or accidental ; fixed in woody 

 fibres, moveable in tracheae, and accidental in all adventitious forma- 

 tions. If, for instance, we take a cutting of any young branchlet, 

 in M'hich no natural bud is distinguishable, and plant half of it in the 

 ground, several adventitious vital points make their appearance, the 

 lowermost of which give out rootlets, and the uppermost leaf-buds. 

 In this case, vital points or centres make their appearance in the vital 

 zone of the cutting, which would never have existed in the natural 

 condition of the branch. Applying this fact to the case of the liga- 

 ture, he thinks it evident that the cambium or elaborated sap, or 

 whatever may be the source of the tumour deposited between the 

 wood and the bark, must assuredly proceed from the leaves towards 

 the root, and meeting with this obstacle, becomes accumulated there ; 

 its tendency to organize itself not being distributed, a zone of Kdven- 

 titious or occasional vital centres soon appears in that point, whose 

 two forces are quickly manifested; the ascending fibres continue to;^/A> 

 extend themselves without impediment, while those which should ^^^^L^^*- 

 have descended, unable to overcome the impediment presented to 

 their further progress, continue to grow, twisting and interlacing 

 themselves, so as to form a tumour. 



Mr. Miers then refers to the differences which Dr. Allemao be- 

 lieves to exist between his theory of the evolution of each fibre in 

 opposite directions upwards and downwards, and that of Gaudichaud, 

 in respect to which he thinks there must be either a misprint or a 

 complete misapprehension of the views of Gaudichaud, who clearly 

 traces the source of each bud, not from the point of external growth 

 (as Dr. Allemao seems to infer), but from the seat of its origin 

 around the medullary sheath, at the nceud vital or point of de- 

 parture of each independent ascending and descending system of 

 vascular fibre. The origin of numerous distinct bud-formations 

 around the medullary sheath, and the extension of ascending spiral 

 vessels and of corresponding descending dotted vessels from each of 

 these separately, are maintained throughout by Gaudichaud in his 

 " Recherches Generales" as an essential part of his theory, and mi- 

 nutely demonstrated in his figures, both in monocotyledonous and 

 dicotyledonous plants. He even forcibly quotes the same circum- 

 No. LXI. — Proceedings of the Lfnnean Society. 



