CHAP. III. ORIGIN OF WOOD. 263 



No doubt aware of most of the difficulties in the way of 

 the common theories of the formation of wood, Du Petit 

 Tliouars, an ingenious French pliysiologist, who had possessed 

 opportmiities of examining the growth of vegetation in tro- 

 pical countries, constructed a theory, which, although in 

 many points similar to the one proposed, but not proved, by 

 his countryman, De la Hire, is nevertheless, from the facts 

 and illustrations skilfully brought by the French philosopher 

 to his aid, to be considered legitimately as his own. The 

 attention of Du Petit Thouars appears to have been first 

 especially called to the real origin of wood by having re- 

 marked, in the Isle of France, that the branches which are 

 emitted by the truncheons of Dracaena (with which hedges 

 are formed in that colony) root between the rind and old 

 wood, forming rays of which the axis of the new, shoot is the 

 centre. These rays surround the old stem ; the lower ones 

 at once elongate greatly towards the earth, and the upper 

 ones gradually acquire the same direction ; so that at last, as 

 they become disentangled from each other, the whole of them 

 pass downwards to the soil. Reflecting upon this curious 

 fact, and upon a multitude of others, which I have no space 

 to detail, he arrived at the conclusion, that it is not merely 

 in the property of increasing the species that buds agree with 

 seeds, but that they emit roots in like manner ; and that the 

 wood and liber are both formed by the downward descent of 

 bud-roots, at first nourished by the moisture of the cambium, 

 and finally imbedded in the cellular tissue which is the result 

 of the organisation of that secretion. Tliat first tendency 

 of the embryo, when it has disengaged itself from the seed, 

 to send roots downwards and a stem and leaves upwards, and 

 to form buds in the axils of the latter, is in like manner 

 possessed by the buds themselves ; so that plants increase in 

 size by an endless repetition of the same phenomenon. 



Hence a plant is formed of multitudes of buds or fixed 

 embryos, each of which has an independent life and action : 

 by its elongation upwards forming new branches and con- 

 tinuing itself, and by its elongation downwards forming wood 

 and bark; which are therefore, hi Du Petit Thouars's opinion, 

 a mass of roots. 



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