CONCLUDING KEMARKS. 109 



subsequently became capable of grasping a support, whichj as we 

 shall presently see, is a great additional advantage. 



From analogous reasons, it is probable that tendrihbearing 

 plants were primordially twiners, that is, are the descendants of 

 plants having this power and habit. T'or the internodes of the 

 majority revolve, like those of twining plants ; and, in a very few, 

 the flexible stem still retains the capacity of spirally twining round 

 an upriglit stick. With sofne the internodes have lost even the 

 revolving power. Tendril-bearers have undergone much more 

 modification than leaf-climbers; hence it is not surprising that 

 their supposed primordial revolving and twining habits have been 

 lost or modified more frequently than with leaf-climbers. The 

 three great tendril-bearing families in which this loss has occurred 

 m the most marked manner are the Cucurbitacese, Passifloracese, 

 and Vitaceae. In the first the internodes revolve ; but I have heard 

 of no twining form, with the exception (according to Palm, S. 29. 

 52) of Momordica halsamina^ and this is only an imperfect twiner. 

 In the other two fauiilies I can hear of no twiners; and the inter- 

 nodes rarely have the poAver of revolving, this power being confined 

 to the tendrils ; nevertheless the internodes of Passijlora gracilis 

 have this power in a perfect manner, and those of the common Vine 

 m an imperfect degree : so that at least a trace of the supposed 

 primordial habit is always retained by some members of the larger 

 tendril-bearing groups. 



On the view here given, it may be asked, Why have nearly 

 all the plants in so many aboriginally twaning groups been con- 

 verted into leaf-climbers or tendril-bearers ? Of what advantage 

 could this have been to them ? Why did they not remain simple 

 twiners ? We can see several reasons. It might be an advantage 

 to a plant to acquire a thicker stem, with short internodes bearing 

 many or large leaves ; and such stems are ill fitted for twining. 

 Any one who will look during windy weather at twining plants 

 will see that they are easily blown from their support ; not so with 

 tendril-bearers or leaf-climbers, for they quickly and firmly grasp 

 their support by a much more efficient kind of movement. In 

 those plants which still twine, but at the same time possess ten- 

 drils or sensitive petioles, as some species of Bigtwnia, Clematis^ 

 and Tropceolum^ w^e can readily observe how incomparably more 

 securely they grasp an upright stick than do simple twiners. 

 From possessing the power of movement on contact, tendrils can 

 be made very long and thin ; so that little organic matter is ex- 

 pended in their development, and yet a wide circle is swept. 



