490 



BOTANY 



and the roots of Maize and other seedlings behave the same way. 

 The influence of water-currents has been called " Kheotropisin." 



Geotropism 



Gravity may act in two ways. First, it acts in a purely mechani- 

 cal way, as in the downward pull exerted upon a branch by the 

 weight of the fruit borne upon it, or the drooping of a flower like a 



Fuchsia or Snowdrop, 

 "" upon its slender stalk. 



A second sort of stimulus 

 is Geotropism. This is 

 the influence exerted by 

 gravity upon the direc- 

 tion of growing organs. 

 Thus, in most plants, 

 roots normally grow 

 downward (are positively 

 geotropic), shoots are neg- 

 atively geotropic, these 

 movements being entirely 

 independent of gravita- 

 tion. Sometimes organs 

 are at one time positively 

 geotropic, at others neg- 

 atively so. This is seen 

 in many nodding flower- 

 buds, like the Poppy. 

 These become erect when 

 the flower opens, so that the seed-vessel stands upright (Fig. 463). 



The growth of a seedling illustrates the geotropic sensitiveness of 

 the root and shoot. If a seedling is placed horizontally, the shoot 

 will quickly bend upward, the root downward. Among the lower 

 plants geotropism is equally manifest. Thus the sporangiophores 

 of Fungi are negatively geotropic, as are the capsules of Mosses or 

 Liverworts, while the rhizoids in all these are positively geotropic. 



That gravity, and not light, is the factor determining the direction 

 of growth of geotropic organs, is shown by placing the plant upon a 

 klinostat, and causing it to rotate slowly upon a horizontal axis, so 

 as to subject all parts equally to the effect of gravity. Under these" 

 conditions no change in the direction of growth can be detected. 



Diageotropism. Certain organs grow normally in a horizontal 

 direction. Such are many rhizomes, stolons, lateral roots and 

 branches, certain flowers, etc. ; these organs are " diageotropic." It 

 is, however, doubtful whether in all cases, e.g. many branches, other 



FIG. 461. Carpotropic movements in Allium Ne- 

 apolitanum. The young inflorescence, A, is 

 positively geotropic; the open flowers, B, are 

 negatively geotropic. (After MACDOUGAL.) 



