7^2 -r-'j MECHANICAL LAWS OF GROWTH. 



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 bundle ; 

 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 

 only useful to upright organs by increasing their rigidity and elasticity, and thus 

 making them more adapted for the erect condition ; while this would be quite un- 

 necessary 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 bend 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 Cle?)iatis integrifolia, Papaver pilosiwi 

 and dubiiim, Geinn 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. 



Since geotropic like heliotropic curvatures take place only during growth^, the 

 position of the parts that will curve in the various organs is known beforehand if the 

 course of their growth is known (see Sect. 17); and conversely the part where 

 growth is at any time taking place may be ascertained by this rule from the fact of 

 its curving. 



From causes which we cannot go into here more in detail, the curvature does 

 not generally take place in the form of an arc of a circle ; but there is in organs 

 of considerable length — whether they curve upwards or downwards — a spot where 

 the curvature is greatest, i. e. where the radius of curvature is least. It would appear, 

 from all that is at present known, that when organs are laid in a horizontal position the 

 strongest curvature is always found at the spot where growth is most rapid. But 

 since in erect stems a piece of considerable length (often 20 cm. or more) is actually 

 growing, a long and flat arc is formed when the stem erects itself from a horizontal 

 position, the maximum curvature of which is at a considerable distance from the 

 apex of the stem. In primary roots, on the contrary, growth exists only in a space 

 of a few mm. from the apex, the maximum increment of growth taking place at 



* Sachs, Experimental-Physiologie, p. 105. 



^ See De Vries, in Arbeiten des Wiirzhurg Bot. Inst., Heft II, p. 229. 



^ It must be noted that some organs, if grown in a normal position and then placed horizontal, 

 begin then to grow like the nodes of Grasses and the contractile organs of Fhaseolus. 



