838 



ORIGIN OF SPECIES. 



themselves to grow upright and round which they twine ; and in order that such a 

 neighbouring support may be more easily and certainly taken hold of, the slender 

 stem of climbing plants is endowed with a power of revolution by which the apex 

 is carried round in a circle and occasionally pressed closely to the stem of an upright 

 plant, up which it then climbs. 



The greater number of plants provided with tendrils are also dependent on the 

 proximity of erect plants round which they can climb ; they are characterised by an 

 extreme parsimony in the employment of organic substances for the purpose of an 

 erect growth. Sometimes (as in the grape-vine) the tendrils are axial structures 

 furnished with minute leaves and branching from the axils of these ; but much 

 more commonly (as in Clematis or Tropseolum) the petioles, or (as in Fumaria) the 

 branched narrowly -divided lamina, or most often the metamorphosed apical parts of 

 the foliage-leaves {Cobcea sca?idens, the pea and other Papilionaceae) are developed in 

 a filiform manner and perform the function of tendrils. The morphological signi- 

 ficance of the tendrils of Cucurbitacese is not yet perfectly determined; but they 

 are probably metamorphosed branches. Tendrils occur only in those plants whose 

 stem is not able to bear in an erect position the weight of the foliage, flowers, and 

 fruits ; in the genus Vicia, for example, all the slender-stemmed species have leaf- 

 tendrils ; but in the thick-stemmed erect V. Faha they are rudimentary. The office 

 of tendrils is to twine round the slender branches and the leaves of other neigh- 

 bouring plants, and thus to fix the apex of the stem as with cords on various sides 

 while it is growing upwards. The mode of development of tendrils, /. e. their 

 endowment with useful properties corresponding to their purpose, is, as Darwin 

 has shown, not only extremely diverse, but exhibits also very different grades of 

 perfection, like climbing stems. Some tendrils are only of slight use ; sometimes 

 (as in some species of Bignonia) they are rather helps to an imperfectly climbing 

 stem ; but where they are perfectly adapted to their function, a variety of properties 

 concur in a remarkable way to increase to a maximum the mode of adaptation to 

 the use of the plant. The tendrils radiate in different directions from the growing 

 apex of the shoot, which makes movements of revolving nutation by which the 

 tendrils are brought into the greatest variety of positions, they themselves also 

 revolving at the same time, so that within a certain area, often not a very small 

 one, they assume an infinite number of positions, by which they must almost 

 inevitably be brought into contact with some support, such as a branch or leaf, 

 lying within this area. The supports are, so to speak, sought out in the most 

 industrious manner ; when one is touched by a tendril, the tendril bends and 

 twines firmly round it ; and when several tendrils do the same in different direc- 

 tion from the stem, it hangs suspended between the points of support. If this 

 were all, the attachment would be a very weak one, and the elevation of the stem 

 would only take place slowly ; but the whole contrivance is perfected in the most 

 ingenious way. When the tendrils have fixed themselves by their extremities, they 

 draw the stem towards the support by twisting themselves spirally. When several 

 tendrils do this in different directions, the stem which is suspended between them 

 is tightly stretched, and the tenacity of the tendrils is at the same time con- 

 siderably increased by the twisting. Many tendrils, while very tender at the time 

 when Ihey are sensitive, become afterwards hard and woody, and some become 



