MOVEMENTS OF IRRITATION. 



495 



curvatures Vochting termed rectipetality (JBewegungen der Bliitlien 

 find Friichte, p. 31), without however thereby professing to make 

 clear the nature of the phenomenon. 



It must also be regarded as a consequence of the co-operation of 

 geotropism in the winding of twining plants, that they cannot coil 

 round horizontal supports, a fact of which we can easily satisfy 

 ourselves by experiment. Bean plants will not wind round sup- 

 ports inclined more than 40. 



The following experiment is also 

 very instructive, showing among 

 other things that the formation of 

 turns in twining plants is quite 

 certainly not the result of contact 

 stimulus. On the end bud of a 

 young pot plant of Phaseolus 

 which has already coiled a few 

 internodes round a support, is 

 fastened a fine thread which runs 

 over a lightly running pulley, 

 brought vertically over the plant. 

 On the free end of the thread we 

 hang a small weight (in my ex- 

 periments, I used a weight of 1 

 gr.) just sufficient to support the 

 stem. After one to two days, the 

 terminal portion of the stem will 

 have formed a number of free 

 turns (see Fig. 165), which how- 

 ever, as the apical growth of the 

 stem continues, gradually dis- 

 appear again, since geotropism, as 

 under other conditions, ultimately 

 causes the internodes to straighten 

 out. If before beginning the ex- 

 periment, we make fine ink-dots 

 along the stem at short distances 

 from each other, it will be found, 

 after the disappearance of the 

 free turns formed in the first 

 place, that the dots are no longer in a straight line, but are 

 arranged on a spiral ascending from the left. Homodromous 



Fig. 165. Phaseolus plant, whose 

 stem above the support has made free 

 turns. 



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