502 LECTURE XIX. 



rhizomes when exposed to light is the resultant of diageo- 

 tropism and of photo-epinasty, the latter being much the 

 more powerful factor. But this suggestion is scarcely to be 

 reconciled with the fact that very faint light, as I have ob- 

 served, suffices to cause the downward curvature. 



The plagiotropism of lateral roots is not, like that of these 

 rhizomes, due to Diageotropism, but depends simply on their 

 slight geotropic irritability (p. 462). They are positively 

 geotropic, but their response to the action of gravity is only 

 sufficient to cause their direction of growth, in uniformly 

 moist soil, to deviate but little from the horizontal. As in 

 the case of primary roots, their hydrotropic irritability is 

 much greater than their geotropic, so that their direction of 

 growth may be considerably affected by hydrotropism. They 

 curve from relatively dry into relatively moist soil, or from 

 dry soil into a saturated atmosphere ; the curvature may be 

 even such that their direction of growth is vertically upwards. 



The plagiotropism of dorsiventral organs, such as shoots 

 and leaves, is a more complicated phenomenon. It is the 

 resultant expression of the effect of light and of gravity upon 

 them, promoted, in many cases, by their own weight In some 

 cases light, and in others gravity, is the determining factor, 

 as is clearly shewn by the fact that some of these organs 

 assume a vertical direction of growth in darkness (p. 441 and 

 469), whereas others remain plagiotropic. The nature of 

 the effect produced by light, appears, so far as we know it, to 

 be in all cases the same : it induces epinasty and diaheliotro- 

 pism (p. 450), that is, it affects not only the direction of 

 growth but also the direction of the opposed surfaces of the 

 organ. As regards the effect of gravity, we see that it is not 

 uniform like that of light, for some of these organs grow erect 

 when gravity is the sole external directive influence acting 

 upon them, whereas others continue to grow horizontally. 

 Plagiotropism is induced by gravity only in the latter case, in 

 organs, namely, which possess that peculiar irritability to its 

 action which we know as Diageotropism (p. 470), in virtue of 

 which these organs tend to direct their morphologically 

 superior surface upwards at right angles to the vertical, a 



