93 Ö ORIGIN OF SPECIES. 
f 
growing rapidly and strongly; a knee-shaped bend is thus formed by which the 
upper part of the haulm is raised up. If, on the contrary, the stem is perennial, and 
has to bear a great weight of branches, leaves, and fruits, contrivances of this kind 
are not sufficient, and then the tissue becomes woody ; if the weight of the crown 
increases year by year, the stem also becomes thicker each year, as in dicotyledonous 
trees and Conifers; if the weight of the foliage does not increase, as in Palms, the 
stem only retains the same thickness. In such cases a considerable quantity of as- 
similated food-material is necessary in order to produce the massive solid stem, while 
in many other cases the elevation is attained at the expense of a very small amount 
of organic substance, as in climbing and twining plants, such as are found in the 
most widely separated families of Angiosperms. Plants with a twining stem Hke the 
Hop presuppose in general the existence and proximity of other plants which are able 
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 is enabled to come into contact with 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 Tropceolum) the petioles, or (as in Fumarid) the 
branched narrowly-divided lamina, or most often the metamorphosed apical parts of 
the foliage-leaves {Cobcea scandens^ the Pea and other Papilionacese) 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 adaptation of tendrils, t. 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 that of 
climbing stems. Some tendrils are only of slight use ; sometimes (as in some 
species of Bignonid) they are merely 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 this kind 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 inta 
contact with some support, such as a branch or leaf, lying within this area. The 
