488 LECTURE XIX. 



as so many springs giving the stem a certain freedom of play 

 when it is swayed by the wind. 



It may be mentioned here that the tendrils of many plants, 

 if they fail to become attached to a support, likewise coil 

 spirally ; but this is a very different matter from the spiral coiling 

 of attached tendrils of which we have just been speaking. It 

 is a much slower process, and it only begins at the time when 

 the tendrils are ceasing to grow and to be irritable : moreover, 

 inasmuch as the spontaneously coiling tendril is attached at 

 one end only, the turns -of the spiral are all in the same 

 direction. In some few cases, as in Bignonia and in three 

 genera of Vitacese (Darwin), this spontaneous coiling of un- 

 attached tendrils does not take place, but they simply remain 

 straight, wither, and fall off. 



Darwin gives the following instances to illustrate the relative time of 

 commencement of the spiral coiling of attached and of unattached 

 tendrils. In Echinocystis, the tendrils usually begin to coil spirally in 

 12 24 hours after being attached, whilst unattached tendrils do not 

 begin to coil until two or three days, or even longer, after circumnutation 

 has ceased. A full-grown tendril of Passiflora quadrangularis which 

 had caught a stick began to coil in 8 hours, and in 24 hours formed 

 several spires : a younger tendril, only two-thirds grown, showed the 

 first trace of coiling two days after clasping a stick, and in two more days 

 formed several spires. Another young tendril, of about the same age 

 and length as the last, did not become attached ; it acquired its full 

 length in four days ; in six more days it first became flexuous, and in 

 two more days formed one complete spire ; the first spire was formed 

 towards the basal end, and the coiling steadily but slowly progressed 

 towards the apex, but the whole was not closely wound up into a spire 

 until 21 days had elapsed from the first observation, that is, until 17 days 

 after the tendril had grown to its full length. 



With regard to the mechanism by which the coiling of 

 a tendril is effected, it may be at once stated that here too, 

 as in all the curvatures which we have hitherto considered, 

 it is due to an elongation of the side which becomes convex ; 

 and in this case also, the general law holds good, that, if the 

 organ is actively growing at the time of curvature, the concave 

 side also elongates, but not to the same extent as the convex 

 side, whereas, if the growth of the organ is slow, the con- 



