GROWTH OP THE STEM. 



29 



the piece that has white wood, they assume the 

 Bame colour. 5thfy, If it be the development 

 of the buds that gives rise to the foi-mation of 

 the wood, how can the first woody layer itself 

 form on a young shoot of the first year, when 

 as yet none of the buds which it supports have 

 been developed? According to the celebrated 

 academician whose theory we are here explain- 

 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 internal struc- 

 ture of the young shoot, we see that from the 

 base of each leaf there proceeds a buftdle of 

 fibres, which, by joining those fi'om 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 form 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 

 phenomena of the growth of vegetables in 

 diameter. That of Du Ham el is essentially found- 

 ed upon the annual transfomiation of the liber 

 into alburntim, 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 i-epeated the experiment 

 after Du Hamel 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 therefore of necessity fall, if 

 we sap the foundation on which its author 

 raised it. The following is the explanation 

 which appears to agree best with facts. 



The annual formation of woody layers is 

 owing to the cambium, which every successive 

 year fonns 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 

 number 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 facts. If we examine a young 

 branch at the period of vegetation, that is, when 

 the sap circulates abundantly in all parts of the 



vegetable, we find the following appearances : — 

 Between the liber and alburnum is seen a layer 

 of a fluid, which, at first colourless and limpid, 

 gradually thickens, and acquires consistence. 

 This fluid, the cambium, is fonned by the de- 

 scending sap, mixed with part of the proper 

 juices of the vegetable. As the cambium thick- 

 ens, filaments are seen to form in its interior, 

 and it is presently 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 fomiation of the annual layer takes 

 place in a slow and progressive manner. It is 

 for this reason that the new layers of alburnum 

 very fi'equently 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 growth in 

 diameter, giving rise each successive year to the 

 fonnation of a layer of alburnum and a laj'er of 

 liber, both distinct from each other, although 

 deriving their origin fi-om the same organ. 

 When Du Hamel found in the alburnum the 

 silver wire which he thought he had inserted in 

 the liber, it was becatise 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 gradually becomes 

 thicker. It is on this account that the liber is 

 found to be formed of several laminte 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 

 formed each year in the ti-unk of dicotyledonous 

 trees. This new layer is produced by a part of 

 the cambium, which is organized and becomes 

 solid. The alburnum formed 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 surface by means of a part of the cambium, 

 which successively fonns new laminae. It is by 

 this mechanism that the growth in thickness of 

 the stems of dicotyledonous trees seems to be' 

 effected. 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 gemmule 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 



