when divided in the Direction of their Axis. 61 



the discoveries of Dutrochet. This acute investigator has 

 shown that the valves of the pericarp of Impatiens Bal- 

 samina (which, when mature, are well known to separate from 

 each other, and become considerably curved, in such a man- 

 ner that the convexity of the curve is on the epidermic, or 

 external, surface of the valve) are composed of a vesicular 

 tissue, so arranged, that the vesicles, or cells, nearest the ex- 

 ternal are considerably larger than those nearest the internal 

 face of the valves; and, consequently, that each individual 

 portion of the vesicular tissue, thus arranged, becomes injected 

 from an accumulation of sap: all the cells, of course, become 

 turgid, and the larger ones occupy, in consequence, a greater 

 space than before. Their complete distention being, however, 

 prevented by the more compact tissue formed by the aggre- 

 gation of the minuter cells, the whole valve assumes a ten- 

 dency to curve in such a manner, that the external portion, or 

 that composed of the larger cells, may occupy the convex part 

 of the curve ; and this tendency to curve is obeyed as soon as 

 the resistance of the opposite valve is removed by a slight 

 touch or otherwise. For a minute and elaborate account of 

 this and other instances of irritability depending upon a 

 similar vesicular structure, I must refer to the essay of M. 

 Dutrochet. (Nouvelles liecherches sur V Endosmose, &c, 1828, 

 p. 57. et seq.) 



This explanation of Dutrochet, for the separation of the 

 valves of the pericarps of the balsam, may be applied, with 

 but a slight modification, to the phenomena before us. If 

 we make a thin transverse section of the stems or petioles of 

 any plants possessing the property of divergence, as of a 

 lamium, or of the common garden celery, and place it in a 

 drop of water under a good microscope, we shall see a con- 

 siderable quantity of vesicular tissue mixed with vessels. The 

 vesicular tissue itself we shall find to be very compact, and 

 composed of very minute cells, nearest the circumference, or 

 external part, and of much looser tissue, made up of larger 

 cells, nearest the axis, or central part, of the stem, or petiole ; 

 presenting a similar anatomical structure to that of the peri- 

 carps of the balsam, although arranged in an inverted di- 

 rection. When, therefore, a stem possessing this structure 

 is in perfect vigour, its vesicular tissue is injected with sap ; 

 the larger cells nearest the axis of the plant become con- 

 siderably distended, and, in consequence, press upon the 

 neighbouring smaller cells; which, resisting this pressure, 

 give to the larger cells a tendency to separate, and occupy a 

 greater space in consequence of this distention ; their sepa- 

 ration being, however, prevented by their intimate lateral 



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