MOVEMENTS OF GROWTH. 



389 



flower stalks of Hyacinthus orientalis, grown in absence of light, 

 are frequently much twisted ; and so also is the hypoccrtyl of 

 Helianthus annuus under similar circumstances, while the corre- 

 sponding organ of seedlings grown under normal conditions 

 exhibit no torsions. If seeds of Helianthus annuus are laid in 

 damp sawdust and cultivated, some in the dark, and some in the 

 light, we can easily satisfy ourselves that this is really the case. 

 It is seen also that the torsions in the etiolated hypocotyls do not 

 appear till towards the end of their growth in length. In many 

 cases torsions are due to internal causes. Others, to which 

 attention may be drawn at once, originate in quite a different 

 manner. The stem of a vigorous marrow plant grown in a pot 

 is tied to a stick in such a way that it cannot execute curvatures. 

 On the upper side of some of the leaf-stalks, and along the mid- 

 ribs of their laminae, we make a line of ink-dots ; and then, after 

 the soil in the pot has been secured by strips of wood, the plant 

 is inverted and placed in the dark. Owing to various causes 

 (geotropism, photoepinastic after-effects) the leaf- stalks, in the 

 course of a few hours, bend upwards ; bat since the weight of the 

 laminae borne by the stalks is hardly ever equally distributed on 

 both sides of the plane of curvature, torsions are produced, of 

 whose extent we can easily judge by observing the ink-marks, 

 which no longer lie in a straight line. These torsions may be 

 rendered permanent by growth. 1 



1 See H. de Vries, Arbeiten d. botan. Inst. in Wiirzburg, Bd. 1, p. 268 ; and 

 Sachs, Lehrbuch d. Botdriik, 1874, p. 833. 



157. Some Examples of Spontaneous Nutations. 



We lay a few soaked seeds of Vicia Faba in loose, damp sawdust^ 

 micro pyle downwards. If we examine oar seedlings carefully 

 just when the stem- begins to emerge between the cotyledons, 

 we shall find that they have straight roots, directed vertically 

 downwards. We now fix a number of Vicia seedlings in a suitable 

 receiver, and exclude the light. At the end of twenty-four hours 

 we find that the roots have departed from their original vertical 

 direction. The roots are curved in the manner represented in 

 Fig. 129, a result due really to a curvature in the hypocotyl and 

 upper parts of the root. The advancing root tip, in consequence 

 of the nutation which has taken place, naturally comes to lie 



