512 MODIFIED CIECUMNUTATION. Chap X 



cut off for a length in three cases of '2 inch and in 

 the foiu- other cases of "14, '12, •!, and -07 inch. But 

 these cotyledons, after being extended horizontally, 

 bowed themselves upwards as effectually as the un- 

 mutilated specimens in the same pots, showing that 

 sensitiveness to gravitation is not confined to their tips. 



Geoteopism. 



This movement is directly the reverse of apogeo- 

 tropism. Many organs bend downwards through epi- 

 aasty or apheliotropism or from tbeir own weight ; but 

 we have met with very few cases of a downward move- 

 ment in sub-aerial organs due to geotropism. We 

 shall, however, give one good instance in the following 

 section, in the case of Trifolium subterraneum, and 

 probjvbly in that of Arachis hypogasa. 



On the other hand, all roots which penetrate the 

 ground (including the modified root-like petioles of 

 Megarrhiza and Ipomoea leptopliylla) are guided in their 

 downward course by geotropism ; and so are many 

 aerial roots, whilst others, as those of the Ivy, appear 

 to be indifferent to its action. In our first chapter the 

 movements of the radicles of several seedlings were 

 described. We may there see (Pig. 1) how a radicle 

 of the cabbage, when pointing vertically upwards so 

 as to be very little acted on by geotropism, circum- 

 nutated ; and how another (Fig. 2) which was at first 

 placed in an inclined position bowed itself downwards 

 in a zigzag line, sometimes remaining stationary for a 

 time. Two other radicles of the cabbage travelled 

 downwards in almost rectilinear courses. A. radicle of 

 the bean placed upright (fig. 20) made a great sweep 

 and zigzagged; but as it sank downwards and was 

 more strongly acted on by geotropism, it moved in an 



