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. Allemäo 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 tracheæ, 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. Allemäo, 
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 sa as to 
form a tumour. Under this point of view, Dr. Allemäo 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, &e., which are each only creme À 
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 cem 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, &e., and ed 
principally spiral vessels; and downwards by means of dotted or scalariform ura s 
towards the roots, producing in their progress depositions of woody fibre, which annually 
increase the diameter of the stems. Dr. Allemão’s general remarks tend to confirm these 
* This same term, “ nœud vital,” was, I believe, first used in 1830 by Turpin (Mém. Mus. -— 16) to irata 
the latent bud, whether existing in the stem, in suckers, or in underground tubers, each “nœud vn rs a = 
to the embryo of the seed, and giving origin to two distinct systems of vessels, one ascending, t e ot a escending. 
He showed that the tubers of the Potato and Topinambour are true subterranean stems, furnished wit sr 
distinct ** nœuds vitaux,” commonly called “ eyes,” which are altogether wanting in the Convolvulus Batatas, the tuber 
of which is simply an expanded root.—J. M. 
