250 The Living Plant 



subject will show, are specialized for cross-pollination in a way 

 which makes the lowermost petals alighting places for insects; 

 and therefore these petals must be kept horizontal. For the 

 same reason the long tubes of Daffodils are geotropically hori- 

 zontal, as one can prove by fastening the young flower-stems in 

 horizontal positions; and there are other cases without number. 

 As to fruits, they are mostly indifferent geotropically, but a few, 

 e. g. Cyclamens and Pea-nuts, use gravitation as a guide as they 

 bury their seeds in the earth. 



So many and interesting are the manifestations of geotropism 

 in special cases that I must take room for a few more examples. 

 Trailing vines, whose main stems rest flat on the ground, like the 

 Periwinkle, Twin-flower, and Ground Pine, and perennials with 

 horizontal stems just beneath it, like Solomon's Seal, keep these 

 positions by virtue of the fact that their main stems have not the 

 usual main-stem geotropism, which is upright, but the trans- 

 verse kind characteristic of side-branches; twining plants are 

 kept encircling a vertical support under guidance of a lateral 

 geotropism, and this is what prevents them from twining around 

 horizontal branches or supports which would not take them up 

 towards the light ; the aerial roots of many tropical climbers, and 

 most tendrils, have likewise this lateral geotropism, which keeps 

 them swinging horizontally until they meet with a support; and 

 there are many other cases of which some may be identified by 

 the reader himself if he keeps observationally alert in his walks 

 abroad in field, garden, or forest. 



Of all of the stimuli made use of by plants for guiding their 

 parts to positions of greatest advantage, gravitation is much the 

 most important. Plants are born with an hereditary tendency 

 to put forth their parts in a symmetrical manner, as can be 

 demonstrated experimentally by aid of the clinostat; but they 

 depend upon geotropism to guide those parts into the suitable 

 positions, and thus to realize the ultimate shape of the plant. 

 And this is the case no matter what the form of the plant may be, 



