230 On the Anatomy of Vegetables. 



thicker. To explain the two phenomena of lengthening 

 and becoming thicker, the aclion of which is fimultaneous, 

 wc nmlt acknowledge that the expaniive force acling in the 

 membranous tiflue newly created is modified by the nature 

 ot the tiffue itfelf. It is compoied, as we have already feen, 

 of two organic elements, one of which is cellular tiflue, 

 formed of the cells, whole diameter is nearly equal in everv 

 direction ; the other is the tubular tiiTuc, formed of the fmall 

 and large tubes contiguous to each other. Let us fuppofe, 

 for a moment, that the fluids imbibed by the vegetable are 

 the caufe of this diflimilarity in the tiflue. We may do fo the 

 more, as this fyftem is not void of probability. By way of 

 example, let us employ the embryo : let us take the feed be- 

 fore fecundation : it is attached to the mother plant by the 

 umbilical cord, the organization of which no doubt varies 

 according to the fpecies. By the help of the veflels which 

 unite this organ to the feed, the fluids penetrate into the or- 

 ganizing fubttance ; and their impulfe being determined by 

 the canals which afford them a paflage, they then trace out 

 the route which the fluids will afterwards purfue, and deter- 

 mine the order of the future developments. Being vigo- 

 roufly puflied in different points, which vary according to the 

 fpecies, they open the longitudinal tubes, and then filter 

 (lowly through their fides, depotit themfelves in the. organiz- 

 ing fubftance, and favour the development of the cells. In 

 the firlt cafe, the fluids are pufhed by the force which gives 

 motion to the lap ; in the fecond cafe, they iflue forth and 

 penetrate the organizing fubftance only, becaufe they tend to 

 aflume an equilibrium. Thefe two forces, balanced by each 

 other, produce a multitude of intermediate fliades between 

 the longitudinal tubes and the perfect cellular tiffue. But 

 this theory is fiill far from explaining the phenomena of ve- 

 getable organization. There doubtlefs exifl athoufand other 

 phyfical caufes, the influence of which cannot be calculated; 

 ana above all thefe caufes ought to be placed the organizing 

 power, the principle of which is totally unknown to us. 



However, the cells and tubes being once formed, they in- 

 creafe till the infpiffation and induration of the membranes 

 prefent an obftacle to their development. During the growth 

 of the membranous tiflue, the fluids carried into the tubes, 

 by feveral forces combined, determine the direction of the 

 elongation bv the impulfe which they give to the organic 

 molcculoe. But the cells fufter themfelves to be penetrated 

 by the fluids only in a flow manner; and, being fubjected 

 to no force which determines their development in one di- 

 rection rather than in another, they grow and dilate them- 



felve£ 



