GROWTH OF THE STEM. 



27 



llie cortic:\l layers and the medullary tuhe. This 

 peculiar Huid contains the first rudiments of or- 

 ganization. In proportion as the young stem is 

 developed, the innermost layer of this fluid ac- 

 ((uires consistence, is organised, becomes hard- 

 ened, and changes into liber, which at the end 

 of the first year, is found to be converted into a 

 soft and half-formed woody substance. Autumn 

 an-ives, and vegetation is arrested in this state. 

 The outer layer of the cambium, which has not 

 yet entirely changed its nature, remains station- 

 ary, and as it were torpid. But, at the return 

 of spring, when the gentle heat of the sun 

 awakens vegetaliles from their winter's sleej), 

 the cambium resumes its vegetative power. It 

 devolopes the buds and the new roots, and, when 

 it has produced all the parts that are to serve 

 for supporting the life of the vegetable, it grad- 

 ually liardens, becomes compact, and undergoes 

 the same changes as that which preceded it. 

 But, in proportion as these changes are effected, 

 as the liber hardens and changes its nature, as 

 the layer which it has replaced acquires greater 

 solidity, a new liber is developed. From all 

 parts of the outer surface of that which is ready 

 to be converted into wood, there exudes a viscid 

 humor, under the form of small drops, which 

 spread and unite. This is a new cambium, a 

 new liber, which is about to be organized, and 

 to pass through the different stages of growth that 

 have been gone through by those which have pre- 

 ceded it, and from which it has derived its origin. 



Such are the means which nature employs for 

 renewing each year successively the vegetating 

 part of the stem. It is here that the greatest 

 difference between woody stems and herbaceous 

 stems presents itself. In woody stems, it is to 

 the successive development of a new layer of 

 liber that the tree owes its duration and the 

 continuance of its vegetation. In herbaceous 

 stems, on the contrary, all the cambium is con- 

 sumed in producing the different organs of the 

 plant, and at the end of the year it is found to 

 be entirely converted into a kind of ligniform, 

 dry, and arid substance. There does not, there- 

 fore, remain, as in the woody stem, a certain 

 (juantity of gelatinous matter, to which is con- 

 fided the charge of preserving, from year to year, 

 the germs of a new vegetation, and the plant 

 necessarily dies, for want of a substance qualified 

 to renew its development. 



Having thus explained the theory of the 

 formation of woody layers by means of the an- 

 nual transformation of the liber into alburnum, 

 we shall next make known the theory which 

 has been proposed by Du Petit-Thouars, and 

 which, to many physiologists, has formed a sub- 

 ject of so much dispute. 



The successive formation of the woody layers, 

 in other words, the growth in diameter, is pro- 

 duced liy the development of the buds. 



In Du Hamel's theory, the liber performs the 

 principal part in the phenomena of the growtli 

 in diameter ; but here the buds are the most 

 importimt instruments in that operation. Du 

 Petit-Thouars having remarked that the buds 

 are seated upon the external parenchyma, and 

 that their fibres communicate with those of the 

 scions or young branches which support them, 

 has drawn from these circumstances the follow- 

 ing conclusions, which form the basis of his 

 theory of vegetable organization. 



1st, Buds are the firet pereeptible phenomena 

 of vegetation. All the parts which in vegetables 

 are to be developed at the exterior, are at first 

 contained in buds. There is one in the axilla 

 of every leaf ; but this bud is apparent in dico- 

 tyledonous plants only, and, among the monoco- 

 tyledones, in the single family of the grasses. 

 In the other monocotyledones, the bud is latent, 

 and consists merely of a vital point, which, in 

 certain circumstances, is susceptible of being 

 developed in the manner of the buds of dicoty- 

 ledonous plants. 



2dly, By their development, buds give rise to 

 scions or young branches, which are furnished 

 with leaves, and most commonly with flowers. 

 Each bud has an existence in some measure in- 

 dependent of that of the other buds. Du Petit- 

 Thouai's considers them as analogous in their 

 structure and development to the embryos con- 

 tained in the interior of seeds, which, through 

 the act of germination, develops a young stem, 

 that may be compared to the scion produced by 

 the evolution of a bud. Accordingly, he has 

 given the name of fixed or adherent embryos to 

 the latter, in opposition to that of free embryos, 

 wliich he applies to those contained in the in- 

 terior of tlio seed. 



Sdly, If we examine the interior of these buds 

 on a scion or young branch of the year, we shall 

 find that they communicate directly with the 

 internal parenchyma or pith. Now, this pith-, 

 as before mentioned, is at first green, and its 

 cellules are filled with an abundance of aqueous 

 fluids. It is from these fluids that the buds 

 derive the first materials for their develop- 

 ment. They are thus nourished at the expense 

 of the intciTial parenchyma, and, by absorbing 

 the fluids wliich it contains, dry it up, and con- 

 vert it into pith, properly so called, wliich is 

 more or less opaque or transparent. 



ithfy. As soon as these buds make their ap- 

 pearance, they obey two general motions, the 

 one ascending or aerial, the other descending or 

 terrestrial. It is here that M. Du Petit-Thouars 

 finds a similarity in the structure and uses of 

 buds to those of the seed-embryos. He considers 

 buds in some measure as germinating embryos. 

 The layer of cambium situated between the bark 

 and the wood is, with respect to the bud, analo- 

 gous to the soil in which the seed begins to 



