462 LECTURE XVIII. 



action of gravity upon an orthotropic organ is greatest when 

 that organ is removed as far as possible from its normal 

 relation to the vertical. 



The discussion of these observations of Elfving's naturally 

 leads us on to the phenomena presented by radial but plagio- 

 tropic organs. We begin with lateral roots. The lateral 

 roots which spring from a tap-root, do not grow vertically 

 downwards like the latter, but nearly horizontally outwards 

 with a downward inclination. It might be supposed from this 

 that lateral roots are not at all geotropic, and that their 

 normal direction of growth is determined simply by their 

 relation to the axis which bears them, by their " proper angle " 

 (see p. 421). They are, however, positively geotropic, as 

 Sachs has shewn. He found that when a pot in which a 

 seedling was growing was turned upside down, and was kept 

 for some time in that position, the lateral roots curved down- 

 wards so as to assume their normal position with regard to 

 the vertical. He found, further, that when seedlings of Vicia 

 Faba were made to rotate with such a velocity that the accele- 

 ration due to the centrifugal force was ^g (g representing the 

 acceleration due to gravity), the lateral roots of the first order 

 curved outwards so that their long axes approached the 

 direction of action of the centrifugal force. He ascertained 

 also that the greater the centrifugal force, the more strongly 

 marked was the curvature, though the curvature did not increase 

 in direct proportion to the acceleration due to the centrifugal 

 force, but in some smaller proportion. 



These lateral roots are clearly organs endowed with low 

 geotropic irritability. Their behaviour is of the same kind as 

 that of the primary roots in Elfving's experiments. Their 

 normal direction of growth is just that which is determined 

 by their feeble positive geotropism. Their plagiotropism is 

 simply the expression of their feeble geotropic irritability, 

 just as the orthotropism of other radial organs is the expression 

 of their high geotropic irritability. 



We come now to certain cases of plagiotropism in radial 

 organs which cannot be explained in this way. Elfving has 

 observed, in the case of the horizontally creeping rhizomes of 



