MOVEMENTS. 113 



Creeper are negatively heliotropie; and they, therefore, 

 turn to walls or trees, to which they attach themselves 

 for support of the plant. 



144. The stems of most of the higher plants grow 

 upwards, or from the earth, and the roots grow downwards, 

 or towards the earth ; the stems of most Mosses grow 

 upwards, and their rhizoids downwards ; the spore-bearing 

 (conidia) filaments or hyphse of some Fungi grow upwards, 

 and the root-like hyphse downwards. To designate these 

 phenomena of growth, the term geotropism (Gr. ge, 

 earth ; trepein, to turn) has been used. The organ is 

 positively geotropic if it grows downwards ; and nega- 

 tively geotropic if it grows upwards. That geotro- 

 pism is due to the influence of gravitation may be 

 demonstrated by placing germinating seeds on rapidly 

 rotating wheels. If the rotation is vertical, the centrifugal 

 force is substituted for gravitation, and the roots grow 

 away from the centre or hub of the wheel, and the stems 

 grow" towards it; if the rotation is horizontal, the cen- 

 trifugal force and gravitation act at right angles, and the 

 roots will grow in a line coinciding with a diagonal, or 

 resultant of the two forces, outwards and downwards, and 

 the stems will grow upwards and inwards. If geotropic 

 organs are placed horizontally, they will curve upwards 

 or downwards, even when considerable resistance is offered. 

 The cells are more elongated upon the convex than upon 

 the concave side ; but how gravitation causes this has not 

 as yet been explained. 



145. A few plants exhibit spontaneous movements 

 of some of their foliar organs. In Desmodium gyrans, a 

 plant of India, with trifoliate leaves, the small lateral 

 leaflets bend continually upon their slender stalks in such 



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