27 



tracts the most forcibly, and by this curvature one valve must neces- 

 sarily be removed from the next, evidently showing that the tissues 

 act as unequal antagonists to each other. 



A very analogous effect can be observed in the species of Gerani- 

 um, which, if examined before the fruit is quite ripe, will be found to 

 have the several carpels attached to the base of an elongated recepta- 

 cle, and the apices of each (styles) to be attached to its whole length. 

 When maturation arrives, by the loss of moisture the lengthened apex 

 of each carpel evinces a tendency to be curved outwards, and this 

 tendency increases to such extent, that ultimately it overcomes the 

 adhesion which existed between the receptacle and itself, excepting 

 at the point, and separates from it with considerable elastic force, 

 and ejects the seed from the carpel through a naturally formed aper- 

 ture, at the same moment when it assumes a curved form, still main- 

 taining adhesion by its apex to the receptacle. 



If this elongated portion of the carpel be examined, either by mak- 

 ing a transverse or longitudinal section, which can be well done in 

 Geranium sanguineum or dissectum, it will be readily seen that the 

 part which curves is composed of two tissues of different colours 

 and densities, the most dense being situated externally and on the 

 concave face of the organ ; the effect being exactly similar to that 

 produced by riveting together a flat piece of brass to a similar piece 

 of steel, and exposing the whole to the same temperature. As these 

 expand unequally when heated together, a curvature is formed, the 

 convexity of which, of course, will be on the side of the most expan- 

 sive metal. 



The elongated beak of the carpel of Er odium cicutarium and other 

 species is, like the Geranium, attached to an elongated receptacle, 

 which it leaves not by a sudden elastic spring, but by the carpel be- 

 coming detached first, and then the apex twisting several times, until 

 at last it separates itself from the receptacle. In the structure this 

 elongated portion of the fruit is somewhat like the Geranium. It pre- 

 sents in the transverse section an elliptical outline, and exhibits on 

 one side a tissue composed of woody fibres, with extremely thick pa- 

 rietes, there being scarcely any canal in their interior, and on the other 

 a still more dense tissue ; at the spot where these tissues unite the fi- 

 bres possess a greater canal and a looser character, which appears a 

 change gradually effected on one side only, for where the fibres pos- 

 sess the greatest calibre, there is no gradual diminution towards the 

 other of their thickness. 



The organ is not composed equally of the two kinds, but one occu- 



