GROWTH OF THE STKM. 



29 



tire piece that has nhite wood, they assume the 

 srnne colour. Stfily, If it be the development 

 of the buds that gives rise to the formation of 

 the Mood, how can the first woody layer itself 

 lonn on a youn? shoot of the first year, when 

 as yet none of the l)uds which it sniiports have 

 been developed ? According to the celebrated 

 academician whose theory we are here exj)lain- 

 ing, at the moment when a bud is developed to 

 form a scion, the leaves which compose it separate 

 from each other, leaving spaces between them. 

 If at this period we examine the intemal struc- 

 ture of the young shoot, we see that from the 

 base of each leaf there proceeds a bundle of 

 fibi-es, which, by joining those from the other 

 leaves, forms the medullary tube ; but as these 

 leaves become developed, there appears in the 

 axilla of each of them a bud, which subsequently 

 tends to establish its radical communication, by 

 shooting forth woody fibres, which gradually 

 cover the medullary tube, and fonn a continuous 

 layer around it. 



The two theories which we have just stated 

 cannot then be adopted in all their parts, as 

 affording a satisfactory explanation of all the 

 )ihenomena of the growth of vegetables in 

 diameter. That ofDuHamel is essentially found- 

 ed upon the annual transfonnation of the liber 

 into alburnum, and its reproduction by means 

 of the layer of cambium. The experiment by 

 which that celebrated naturalist having passed 

 a silver wire into the liber, found it the follow- 

 ing year in the alburnum, is altogether incon-ect: 

 none of those who have repeated the experiment 

 after Du Ilamel have obtained the same result ; 

 and when the silver wire had actually been 

 passed through the liber, it was always found 

 again in that organ, and not in the alburnum. 

 This theory must thereibre of necessity fall, if 

 we sap the foundation on which its author 

 raised it. The following is the explanation 

 ^vhich appears to agree best with facts. 



The annual formation of woody layers is 

 owing to the cambium, which every successive 

 yearfonns at once a new layer of alburnum and 

 a new layer of liber. 



This is the opinion which Mirbel has latterly 

 professed, and which appears to have the greatest 

 lunnber of probabilities in its favour. 



The liber, hitherto considered as the most 

 essential organ of vegetation, and that which 

 contributes each year to the increase in diameter 

 of the trunk of dicotyledonous trees, being, on 

 the contrary, neutral and passive in this opera- 

 tion, another explanation of the phenomena of 

 growth in diameter must be sought for. The 

 following, then, is that which seems the most 

 probable, and the most conformable to the strict 

 observation of fiicts. If we examine a young 

 branch at the period of vegetation, that is, when 

 tile sai^circulates abundantly in all parts of the 



vegetable, we find the following nppcarancen :— • 

 Between the liber and alburnum is seen a layer 

 of a fluid, which, at fii'st colourless and limpid, 

 gradually thickens, and acquires consistence. 

 This Hind, the cambivm, is formed by the de- 

 scending sap, mixed with jiart of the jiroper 

 juices of the vegetable. As the cambium thick- 

 ens, filaments are seen to fonn in its interior, 

 and it is j)rescntly organized, and assumes the 

 appearance of a vegetable tissue. This transfor- 

 mation is gradual, and continues during the 

 whole period of the development of the buds, 

 so that the fonnation of the annual layer takes 

 place in a slow and progressive manner. It is 

 for this reason that the new layers of alburnum 

 veiy frequently present several concentric zones, 

 which show that their whole thickness has not 

 been formed at once. 



The alburnum is not therefore fonned by the 

 liber, which thickens and acquires more con- 

 sistence, but by the cambium, which is organ- 

 ized, and thus becomes the agent of giowth in 

 diameter, giving rise each successive year to the 

 formation of a layer of alburnum and a layer of 

 liber, both distinct from each other, although 

 deriving their origin from the same organ. 

 When Du Ilamel found in the alburnum the 

 silver wire which he thought he liad inserted in 

 the liber, it was because he had passed the wire 

 through the organic layer of the cambium. 



It also follows from this, that the liber in- 

 creases every year in thickness, by its inner 

 surface ; for the layer of cambium, which bathes 

 its inner surface, becomes organized, and is added 

 to the liber, so that the latter gi-adually becomes 

 thicker.' It is on this account that the liber is 

 found to be formed of several lamina; or leaves, 

 which are connected with each other by an ex- 

 cessively thin layer of cellular tissue. 



In this manner, then, a new woody layer is 

 fonned each year in the trunk of dicotyledonous 

 trees. This new layer is produced by a part of 

 the cambium, which is organized and becomes 

 solid. The alburnum fonned the preceding 

 year acquires more density, and changes into 

 wood. But the liber undergoes no transforma- 

 tion ; it is merely renewed and increased at its 

 inner sui-tace by means of a part of the cambium, 

 which successively fonns new lam i me. It is by 

 this mechanism that the growth in thickness of 

 the stems of dicotyledonous trees seems to be 

 eifected. We shall now explain their develop- 

 ment in height. 



Growth in height. At the period of germina- 

 tion, the radicle sinks into the ground, while 

 the ascending genunule shoots upwards. The 

 first layer of cambium becomes organized, and 

 obeys this impulse. Towards autumn, when it 

 is organized into alburnum and liber, its growth 

 stops. When, at the return of spring, vegeta- 

 tion recommences, the vegetable tissue is gorged 



