HABITS OF CLIMBING PLANTS. 165 



superfluous. Nevertheless I do not wish to assert that they 

 are never irritable ; for the growing axis of the leaf -climbing, 

 but not spirally twining, LopJiosper'mum scandens is, as we 

 shall hereafter see, certainly irritable ; but this case gives me 

 confidence that ordinary twiners do not possess this quality, 

 for directly after putting a stick to the Lophospermum, I saw 

 that it behaved differently from any true twiner or any other 

 leaf-climber. 



" The belief that twiners have a natural tendency to grow 

 spirally probably arose from their assuming this form when 

 wound round a support, and from the extremity, even whilst 

 remaining free, sometimes assuming this same form. The free 

 internodes of vigorously growing plants, when they cease to 

 revolve, become straight, and show no tendency to be spiral ; 

 but when any shoot has nearly ceased to grow, or when the 

 plant is unhealthy, the extremity does occasionally become 

 spiral. I have seen this in a remarkable degree with the ends 

 of the shoots of the Stauntonia and of the allied Akebia, 

 which became closely wound up spirally, just like a tendril, 

 especially after the small, ill-formed leaves had perished. The 

 explanation of this fact is, I believe, that the lower parts of 

 such terminal internodes very gradually and successively lose 

 their power of movement, whilst the portions just above move 

 onward, and in their turn become motionless ; and this ends in 

 formiug an irregular spire. 



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

 rather more slowly than it revolves. For instance, a shoot of 

 the Ceropegia took 9 hours and 30 minutes to make one com- 

 plete spire round a stick, whilst it revolved in 6 hours ; Aris- 

 tolochia gigas revolved in about 5 hours, but took 9 hours and 

 15 minutes to complete its spire. This, I presume, is due to 

 the continued disturbance of the moving force by its arrest- 

 ment 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 

 up 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 revolu- 



