VESSELS IN MONOCOTYLEDONOUS AND DICOTYLEDONOUS PLANTS. 263 



by the depositions left by the descending sap from the woody fibres, where its progress is 

 thus stopped. Hence the question arises, how is it that fibres ascending from the collar 

 of the root create this deposit above, and not below the impediment ? Dr. Allemao thinks 

 this may be accounted for by reasoning on the facts established in the preceding memoir, 

 viz. that in the development of the vascular fibres observed in the stem, there always 

 exists a vital centre, whence they extend themselves in two opposite directions. Now 

 this vital centre*, or central point in the formation of fibre, may be fixed, moveable, or 

 accidental : fixed in woody fibres, moveable in tracheae, and accidental in all adventitious 

 formations. If, for instance, we take a cutting of any young branchlet in which no 

 natural bud is distinguishable, and plant half of it in the ground, several vital points 

 that may be considered adventitious make their appearance, the lowermost of which will 

 give out rootlets, and the uppermost leaf-buds. Is it not therefore clear, asks Dr. Allemao, 

 that in the " vital zone " of this cutting, vital points or centres appear, which would never 

 have existed in the natural condition of the branch ? Applying this fact to the case of 

 the ligature before mentioned, it is evident that the cambium or elaborated sap, or what- 

 ever be the source of the tumour deposited between the wood and the bark, must assuredly 

 proceed from the leaves toward the root, and meeting with this obstacle, becomes accu- 

 mulated there : its tendency to organize itself not being distributed, a zone of adventi- 

 tious or occasional vital centres appears in that point, whose two forces are soon mani- 

 fested ; the ascending fibres continue to extend themselves without impediment, while 

 those which should 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. Under this point of view, Dr. Allemao concludes that his principle of a 

 vital centre is established. 



I cannot perceive any essential difference in these conclusions from the views of Gaudi- 

 chaud, who contends that all the various organs of plants spring from the development 

 of buds generated around the central medullary sheath of the stem, producing by their 

 extension beyond the surface of the stem, leaf-buds, out of which proceed leaves, scales, 

 calyces, corollas, stamens, carpels, ovules, cotyledons, &c, which are each only so many 

 modifications of one original vegetation — the phyton. These buds exist either in an 

 active or a passive state, and being in the latter case only rudimentary, they often remain 

 in the embryo-state of an organized cell, which may at any time under certain circumstances 

 become active. He shows that each active bud has a development of its own, expanding 

 in two opposite directions ; upwards to form fresh leaves, inflorescence, &c, and exhibiting 

 principally spiral vessels ; and downwards by means of dotted or scalariform vessels 

 towards the roots, producing in their progress depositions of woody fibre, which annually 

 increase the diameter of the stems. Dr. Allemao's general remarks tend to confirm these 



* This same term, " noeud vital," was, I believe, first used in 1830 by Turpin (Me'm. Mus. xix. p. 16) to express 

 the latent bud, whether existing in the stem, in suckers, or in underground tubers, each " noeud vital " being analogous 

 to the embryo of the seed, and giving origin to two distinct systems of vessels, one ascending, the other descending. 

 He showed that the tubers of the Potato and Topinambour are true subterranean stems, furnished with numerous 

 distinct " neeuds vitaux," commonly called " eyes," which are altogether wanting in the Convolvulus Batatas, the tuber 

 of which is simply an expanded root. — J. M. 



