672 



LECTURE XXXVIII. 



u 



since they show that twining plants are able to make spiral curves, even without 

 a support, very similar to those they make when twining round a support ; 

 only it is necessary in this case that they are by some means maintained in an 

 upright position. Hugo de Vries (1873) fastened a fine thread to the terminal 

 buds of the freely sweeping shoot -apices of Kidney Beans and various other twining 

 plants [Pharbitis hederacea and Quamoclit luieola), and, by 

 means of a small weight of 2-3 grams, carried the thread 

 over a pulley so that the apex of the shoot was drawn 

 vertically upwards. In the course of a few days spiral turns 

 were developed, and after the portion of the apex here under 

 consideration vs'as fully grown, he observed torsions on it at 

 the same time (though the number of both was such that 

 no causal connection between them could be established), 

 Just as if the shoot had twined round a rod. De Vries also 

 fixed a revolving shoot-apex by gumming to a rod the side 

 which was posterior during the revolution, and obtained a 

 half to a whole spiral turn. 



The experiment with the thread is particularly instructive, 

 and I have also obtained excellent results with it : it suffices 

 to employ a weight just sufficient to pull the thin shoot-apex 

 upright. With Polygonum dumetorum. and Apios tuberosa I 

 obtained in this manner in 15-16 hours 1-2 complete spiral 

 turns, as well as torsions. In these experiments, as De Vries 

 noticed, the essential point is only the prevention of nutations 

 — i. e. of the circular sweeping movement of the apex. There 

 are, however, various other causes which promote the develop- 

 ment of free turns in the absence of a support — above all, the 

 continued upright position of the growing shoot. This may 

 be elegantly demonstrated by cutting off the apical portions 

 20-30 cm. long of shoots of the Hop, the Red Bindweed 

 {Ipomcea purpurea), Menispermum canadense, Dioscorea ba- 

 tatus, &c., which have been grown in the open, and as yet 

 have not attached themselves to supports, but are nearly 

 straight, or curved into the form of a long S, and placing them 

 in a glass cylinder 30-40 cm. high and 5-8 cm. in diameter, 

 at the bottom of which are a few cubic centimetres of water. 

 Under these circumstances the shoots grow actively and 

 elongate 5-10 cm., and in the course of 2-3 days there are 

 developed 2-4 complete spiral turns, which present exactly 

 the same appearance as if the shoot had wound itself round a rod of 1-5-3 '^^• 

 diameter. Here also the upper parts of the coiled shoot (cf. Fig. 380) are almost or 

 quite horizontal. The further down the coiled parts of the shoot lie, and the older 

 they are, the steeper and narrower they are, exactly as if the shoot had twined round 

 a thin support. No essential difference whatever is to be found. If the glass cylinder 

 containing such a shoot is now simply laid horizontally on the table, and rotated 

 through 90° about every hour, so that every side of the spiral shoot-apex is in turn 



FIG. 380.— Three coils, which 

 have been developed indepen- 

 dently of the support, of a very 

 vigorous shoot of Menispermum 

 caitadenset which had been cut off 

 and placed in a glass cylinder* 

 The coils are represented exactly, 

 but the leaves, flowers, and hairs 

 are omitted : the line/'^ is simply 

 to render the direction of the coils 

 evident. 



