130 Introduction to Botany. 



The growing apices of roots and shoots are almost con- 

 stantly in motion, describing more or less irregular ellipses. 

 Such movements are usually so slight as to be inappreci- 

 able without the use of instruments for measuring move- 

 ments through minute distances. 



101. Twining Plants. The rotating movements of the 

 apices of twining plants differ from spontaneous move- 

 ments, not only in degree, but also in kind, for they depend 

 on gravity for their accomplishment. This is shown by 

 Fig. 66, which illustrates a morning-glory that has grown 

 up a support under normal conditions, and the same plant 

 after it has been revolving on a horizontal axis and on a 

 vertical axis at the same time for a period of about eight 

 hours. In the latter instance the plant not only ceased to 

 twine, but actually untwined as far back as growth in 

 length was still taking place. The revolution of a plant 

 on both horizontal and vertical axes at the same time elimi- 

 nates the directive influence of all exterior forces ; but that 

 it is the elimination of the influence of gravity, and not 

 that of light, which has caused the plant to untwine is 

 shown by the fact that under otherwise normal conditions 

 it continues to twine in darkness. 



102. Method of Twining. Most twining plants twine 

 contrary to the movement of the hands of a watch, that 

 is, the coil facing the observer passes from the left below 

 to the right above. Some plants, however, twine in the 

 opposite direction, and some twine indifferently in both 

 directions. The seedling twining plant does not show a 

 tendency to twine for the first one or two internodes, but 

 after that the apical portion bends over by its own weight 

 to a position more or less horizontal, and then, if it is a 

 left to right twiner and the apex is bent toward the north, 

 for instance, the east side of the stem begins to grow faster 



