284 



COMPENDIUM OF GENERAL BOTANY. 



tion in mechanics and mathematics. The study of this problem 

 was begun by VON MOHL and PALM, and continued by CHARLES 

 DARWIN and H. DE VRIES. Great advances in this study have 

 recently been made by the investigations of SCHWENDENER, 

 BARANETZKY, and AMBRONN. The following explanation is based 

 upon the results of AMBRONN'S and SCHWENDENER'S investigations. 

 The active factors in twining are (1) circumnutation of the 

 growing stem-apex and the resistance of the support ; (2) nega- 

 tive geotropism. Both factors aid each other in their effects. 

 Circumnutation makes seizure of or contact with the support 

 possible ; subsequently it is necessary that the contact-stimulus 

 should continue upward and that the curvature should be con- 

 tinuous for a time at least. The advancing of the contact-point 

 is induced by the return pressure of the sup- 

 port. The support exerts a radial pressure 

 outward against the point of contact behind 

 a (Fig. 171), which necessarily increases and 

 extends the contact area at ft. Antidromic 

 torsions prevent a stem-portion as at y, which 

 must become somewhat elongated on the side 

 facing the support due to the pressure of 

 the support, from elongating equally on the 

 outer side when it comes into the position /?. 

 Negative geotropism causes the horizontally 

 or diagonally placed apical stem-portion to 

 curve upward and again brings it in contact 

 with the support at some point higher up, when the effects of 

 the pressure will again come into play, as has just been ex- 

 plained. Negative geotropism also assists in another way in 

 forming permanent spirals. It causes curvatures by the more 

 active growth of a continuous tissue-portion which describes a 

 homodromic spiral line around the stem. 



We have yet to mention the influence of apparent 1 and real 

 torsion in the process of twining. According to Ambronn and 

 Schwendener, apparent torsion causes spiral curvatures, which 



FIG. 171. 



1 Au apparent torsion may be represented by a cylindrical staff cut into many 

 sections, fastened together by spirally-arranged hinge- joints, which are wound 

 about some support. In the case of twining true torsions are caused by the lat- 

 eral pressure of the support acting upon a diagonal portion of the stem. (See 

 Fig. 171.) 



