ACTION OF GRAVITATION ON GROWTH IN LENGTH. 
«43 
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, 7nutatis 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 oi Phaseohis^ 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 integrifolia^ Papaver pilosuin 
and dubimn, Geum rivale, and Anemone praterisis. 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-Physiologie, p. 105. 
^ See De Vries, in Arbeiten des Bot. Inst. Würzburg, Heft II. p. 2 29. 
^ [Elfving (Ueb. einige horizontal-wachsende Rhizome, Arb. d. bot. Inst, in Würzburg, II 3, 
1880) has found that the rhizomes of certain plants {Heliocharis palustris, Spargaiimm ramosum, Scirpiis 
