I570 A TEXTBOOK OF THEORETICAL BOTANY 



without dispersal, by means of movements of the pedicels. The pheno- 

 menon is called carpotropy and it may be differentiated as geocarpy or 

 hydrocarpy respectively, according to the place of growth. 



There are numerous examples both among familiar and unfamiliar 

 plants. Among the first we note Arachis hypogaea, the Ground Nut, 

 already described (p. 1140), in which the " gynophore " elongates while 

 the legume is still small, thrusting it downwards into the soil alongside 

 the roots, where it ripens. Although always called the gynophore, the 

 long carpotropic stalk in Arachis is really the elongated base of the ovary 

 itself, which is quite sessile and has no gynophore in the proper sense of 

 the word, which implies a true floral internode. 



A comparable and very interesting case is that of TrifoUiim subterraneum, 

 where the pedicels bend geotropically after fertilization and thrust the heads 

 of flowers below ground. Only the lowest flowers of the inflorescence are, 

 however, fertile, the rest are sterile and so imperfect that, while the lower 

 ones have some calyx lobes spreading like the flukes of an anchor, the 

 uppermost are merely stumps crowded into a head. These sterile flowers 

 have been regarded both as anchors and as organs of absorption; which 

 they really are, is not clear. In Arachis those fruits which fail to bury 

 themselves do not generally develop, but the TrifoUum species does develop 

 some aerial fruits as w^ell as those underground. It is notable that the 

 former are hard-shelled and delayed in germination, while the latter are 

 soft-shelled and germinate the same season. This differentiation is pro- 

 bably the chief biological advantage of the habit to the species. 



Several other Papilionaceae have the geocarpic habit, among them the 

 cultivated Voandesia stibterranea, the Bambarra Groundnut. The plant is 

 stoloniferous and the flowering branches grow downwards from the 

 stolons, the flowers being produced underground. The pedicels lengthen 

 and carry them up to the surface before pollination, then contract and draw 

 them down again to ripen the one-seeded fruits. 



In contrast to the foregoing it is often the pedicel of the individual 

 flower which becomes geocarpic after fertilization. Linaria cymbalaria 

 grows on walls and rocks and holds its flowers outwards and well exposed 

 until they are pollinated. The phototropic reaction of the pedicels is then 

 reversed and they turn inwards, pushing the young fruits into cracks and 

 crevices, where they ripen and where the seeds are shed. The various 

 species of Cyclamen show a very curious action, for the pedicels not only 

 become geotropic but coil themselves into tight spirals by whose pressure 

 the fruits are forced against the soil. They are too large to be buried 

 directly but they are soon covered by the action of rain on the soil (Fig. 143 1). 



An example of hydrocarpy is given by Eichhornia speciosa, the 

 Water Hyacinth, whose inflorescence axis bends by the curvature of a 

 thickened, pulvinar region and plunges the fertilized flowers below water 

 to ripen the fruits (Fig. 1432). 



Amphicarpy is a term applied especially to plants which have subter- 

 ranean cleistogamous flowers and aerial chasmogamous flowers, pro- 



