SPIRAL TWI>'EES. *^ 11 



When a revolving shoot strikes a stick, it winds round it rather 

 more slowljc than it revolves. Tor instance, a slioot of the Cero- 

 pegia took 9h. 30 m. to make one complete spire round a stick, 

 whilst it revolveil in G h. ; Aristolochia gigas revolved in about 5 h., 

 but took 9 h. 15 m. to complete its spire. This, I presume, is due to 

 the continued disturbance of the moving force by its arrestment 

 at each successive point ; we shall hereafter see that even shaking 

 a plant retards the revolving movement. The terminal internodes 

 of a long, much-inclined, revolving shoot of the Ceropegia^ after 

 they had wound round a stick, always slipped uj) it, so as to 

 render the spire more open than it was at first ; and this was 

 evidently due to the force which caused the revolutions being 

 now almost freed from the constraint of gravity, and allowed 

 to act freely. With the Wistaria, on tlie other hand, a long 

 horizontal shoot wound itself at first in a \qvj close spire, which 

 remained unchanged; but subsequently, as the shoot grew, it 

 made a much more open spire. With all the many plants which 

 were allowed freely to ascend a support, tlie terminal internodes 

 made at first a close spire ; and this, during windy weather, well 

 served to keep the shoots in contact witli their support ; but as 

 the penultimate internodes grew in length, they pushed them- 

 selves up for a considerable space (ascertained by coloured marks 

 on the shoot and on the support) round the stick, and the spire 

 became more open. 



It follows from this latter fact that the position occupied by 

 each leaf with respect to the support, in fact, depends on the 

 growth of the internodes after they have become spirally wound 

 round it. I mention this on account of an observation by Palm 

 (S. 34), who states that the opposite leaves of the Hop always 

 stand exactly over each other, in a row, on the same side of the 

 supporting stick, though this may differ in thickness. My sons 

 visited a hop-field for me, and reported that though they gene- 

 rally found the points of insertion of the leaves over each other 

 for a space of two or three feet in height, yet this never occurred 

 up the whole length of a pole, the point of insertion forming, as 

 might have been expected, an irregular spire. Any irregularity 

 in the pole entirely destroyed the regularity of position of the 

 leaves. From casual inspection, it appeared to me that the op- 

 posite leaves of Thunhergia alata were arranged in a line up the 

 sticks round which they had twined; accordingly I raised a 

 dozen plants, and gave them sticks of various thicknesses and 

 string to twine round; and in this case one alone put of the 



