GEOTROPIC CURVATURE 115 



marked off from the thin cylindrical parts and 

 usually coloured differently. One of these 

 knots with a portion of the stem above and 

 below, comprises a ' haulm. 1 



'If a haulm be bent sharply above the soil, 

 while growing, so that the whole of its upper 

 part comes to lie horizontally, it will be noticed 

 after from two to four days that a knee-like 

 curvature has been formed at its node, in con- 

 sequence of which the apical portion of the 

 haulm again erects itself vertically ; as a rule 

 two or three nodes take part in this change, 

 the nodes being about the knee-like formations. 

 The result is known as geotropic curvature. 

 The curvature of the node is due to the fact 

 that its under side, when placed in the un- 

 wonted horizontal position, becomes consider- 

 ably elongated as a result of vigorous growth, 

 while the upper side grows feebly or not at all, 

 or even often becomes considerably shortened. 



* If a scarlet runner growing erect in a vessel 

 be inverted and left upside-down for some hours, 

 it will, even in that short time, show geotropic 

 curvature of the mobile organs.' (Sachs.) 



