26 



HISTORY OF THE VEGETABLE KINGDOM. 



than at the period of its first formation, and the 

 inserted portion elongating only in consequence 

 of the accumulation of the new layers, by which 

 the diameter of the trunk is increased. In its 

 width, however, it increases like the external 

 portion, by the addition of new layers pervading 

 the alburnum of the trunk, to which it is in- 

 timately united by the intermixture of their 

 respective fibres, forming a finii and compact 

 knot ; this may be seen by cutting across a fir 

 tree immediately above or below a branch ; for 

 the branches are not formed merely by means of 

 a horizontal extension of the longitudinal tubes 

 of the trunk, but are each as it were a distinct 

 individual, of which the external cone is the 

 trunk, and the internal cone the root. Hence, 

 the trunk is to the branches what the soil is to 

 the plant, the source of its nourishment and 

 stability. The branches of trees assume almost 

 all varieties of position, from the reflected to 

 the horizontal and upright ; but the lower 

 branches of trees are said to be generally par- 

 allel to the surface of the soil on which they 

 grow, even though that surface should be the 

 sloping side of a hill, owing, as it has been 

 thought, to the growth of a greater number of 

 buds on the side that forms the obtuse angle 

 with the soU, in consequence of its being exposed 

 to the action of a greater mass of air. 



CHAP. VII. 



GROWTH OF THE STEM. 



All vegetables grow in diameter. It is suffi- 

 cient to cast our eyes on the trees which vege- 

 tate around us, to be convinced of this truth ; 

 nor has any person ever denied it. But by what 

 mechanism is this growth effected? On this 

 point there is the greatest disagreement. Of the 

 different opinions which have been advanced by 

 physiologists, we shall select the three most im- 

 portant, which are these : — 1. Growth is effected 

 by the annual transformation of liber into albur- 

 num ; 2. By the development of buds ; 8. By 

 the cambium, which every year forms a distinct 

 layer of liber and alburnum. In the first place, 

 it has been stated that the growth in diameter 

 is effected, in dicotyledonous trees, by tlie an- 

 nual transformation of the liber into alburnum, 

 of the alburnum into wood, and by the succes- 

 sive renewal of the liber. Such is the founda- 

 tion of Du Ilamel's theory, which that cele- 

 brated author has given at length in his natural 

 history of trees. 



We shall take the stem at the period of its 

 first development, that is wlien, in consequence 

 of germination, it emerges from the seed which 

 contained it, and begins to appear externally. 



All parts of the vegetable that are contained 

 in the seed, previous to germination, are formed 

 exclusively of a dense and regular cellular tissue. 

 The stem, like the other organs, is found to bo 

 entirely destitute of vessels. Properly speaking, 

 there are perceived no traces of bark, pith, lil)er, 

 &c. But scarcely has germination commenced — • 

 scarcely has the stem begun to shoot up, when 

 we see tracheie, false tracheae, and porous vessels 

 forming, and by their union constituting the 

 walls of the medullary tube. This internal part 

 of the stem is the first that is apparent and be- 

 comes organized. The pith is contained within 

 it ; but it is as yet green, and filled witli watery 

 fluids. The outer surface of the medullary tube 

 is soon observed to become covered with a fluid 

 cellular tissue. This is the first layer of cam- 

 bium, which, on the one hand, forms the 

 first liber, and, on the other, constitutes the 

 cortical layera. This liber is presently to be 

 converted into alburnum, in proportion as "a new 

 layer becomes organized to replace the first. The 

 following j'ear, the new liber fonns a second 

 zone of alburnum, and thus successively, each 

 year, a layer of alburnum is converted into true 

 wood, while the liber itself acquires the proper- 

 ties and nature of alburnum. This regular de- 

 velopment of the stem explains the formation 

 of the concentric layers or zones, which are ob- 

 served on a transverse section of the stem of a 

 dicotyledonous tree. But these layers are not 

 all of the same thickness, and frequently the 

 thickness is not equal in the whole circumference 

 of the same layer. An attentive observation 

 easily explains this singular disposition. It has 

 been remarked, that the greatest thickness of 

 the woody layers always corresponds to the side 

 on which the largest roots are found, and which 

 have consequently extracted a more abundant 

 nutriment from the earth. Thus, trees that are 

 situated on the edge of a wood always have 

 thicker woody layers on their outer side, be- 

 cause the roots, meeting no obstacles, extend 

 themselves farther in that direction than in any 

 other, and acquire a larger size. 



In this theory of Du Hamel's, we see that 

 the liber performs the most important part in 

 the formation of the woodj' layers, it being each 

 successive year converted into a new layer of 

 alburnum, which is added to those previously 

 existing. 



The liber being the essential organ of vegeta- 

 tion, and changing its form and consistence each 

 year, it was necessary that nature should also 

 have provided means for reproducing it annually. 

 This, in fact, is the case. If we examine atten- 

 tively the successive development of the dif- 

 ferent organs which compose the stem of the 

 dicotyledones, we see that in the first year, a 

 gelatinous fluid, to which Grew and Du Hamei 

 have given the name of camW«m, occurs between 



