ACTION OF GRAVITATION ON GROWTH IN LENGTH. 843 



A lines are drawn parallel both to the straight and the curved outlines, and the septa 

 of the cells are then indicated in the straight piece simply by parallel lines crossing 

 the first at right angles, in the curved part by lines corresponding to the radii of 

 curvature. The cells exposed by longitudinal sections through nodes of Grasses 

 and roots endowed with geotropic curvature exhibit this phenomenon, although 

 with many irregularities. 



Now that the facts connected with the geotropism of the cell-wall have thus 

 been made clear, we may proceed to the question, why or by what effect of gravita- 

 tion these differences are occasioned in the growth on the upper and under sides of 

 every cell of a geotropic organ when placed in a horizontal position. We have 

 at present however no answer to this question, any more than in the case of helio- 

 tropism, the same diagram availing, mutatis mutandis, for the two phenomena. 



The view brought forward by Hofmeister, and for some time adopted by me, 

 that positive geotropism occurs only in those organs and in those parts of organs 

 in which there is no tension in the tissues, while the organs in which there is strong 

 tension are negatively geotropic, rested on imperfect induction. On the one hand, 

 the parts of the roots of seedlings which curve downwards (as I have shown else- 

 where) are not entirely without tension between the cortex and the axial fibro- 

 vascular cylinder ; while, on the other hand, in the nodes of Grasses, although they 

 display a high degree of negative geotropism, there is no or very little such tension. 

 Even in the negatively geotropic contractile organs of the petioles of Phaseolus^ the 

 tension between the cortex and the axial bundle is of a similar character to that 

 which occurs in positively geotropic roots, but extremely intense. If therefore the 

 tension of tissues and the alteration effected in it by the influence of gravitation 

 cannot be considered as the cause of the upward curvature, it may still be admitted 

 that it is useful to upright organs by increasing their rigidity and elasticity, thus 

 making them more readily assume the erect position ; while this would be quite 

 unnecessary in those that grow downwards. 



A good illustration of the part played by rigidity and elasticity in producing the 

 erect position of negatively geotropic organs is afforded by the pendent pedicels of 

 many flowers and flower-buds, in which the tendency to curve upwards is altogether 

 obscured, the weight of the flower being sufficient to bend the pedicel downwards. 

 If in such cases the flower-buds are cut off, the pedicel becomes erect ^ from the 

 stronger growth of the under side, as e.g. in Clematis integri/olia, Papaver pilosum 

 and dubium, Geum rivale, and Anemone pratensis. The tension in the tissue of such 

 pedicels is not sufficient to give them the rigidity needful to overcome the weight 

 of the flower by their geotropic curvature upwards; this weight, on the contrary, 

 overcomes the tendency of the pedicel to curve convexly on the lower side, which 

 tendency comes into play when the weight is removed. The same is the case in very 

 long but not very rigid shoots, as those of the Weeping Willow, Weeping Ash, &c. 



If a number of organs grow in a horizontal or oblique direction without curving 

 either upwards or downwards, this may result from their not being geotropic'' and 



» Sachs, Experimental-Physiologic, p. 105. 



* See De Vries, in Arbeiten des Bot. Inst. Wurzburg, Heft II. p. 229. 



^ [Elfving (Ueb. einige horizontal-wachsende Rhizome, Arb. d. bot. Inst, in Wurzburg, II 3, 

 1880) has found that the rhizomes of certain plants {Heliocharis palustris, Sparganium ramosum, Scirpus 



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