RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 



417 



almost horizontally. If the surface of the foliage-leaves on the horizontal twig 

 retains the same direction as those on the erect twig here represented, this will 

 be the most disadvantageous position imaginable with regard to the incident light. 

 It is urgently necessary that they should alter this position and again arrange 

 themselves suitably. This rearrangement of the leaf-surfaces proceeding from 

 the horizontal twigs is carried out, and, indeed, in four different ways. Either 

 an adequate twisting of the internodes is effected; or a twisting of the leaf-stalks 

 occurs; or the leaf -stalks do not undergo actual torsion, but their inclination 

 to the leaf -blade becomes altered; or, finally, individual leaf-stalks lengthen to an 

 extraordinary extent; so that the blades borne by them are carried far beyond 



Fig. 107.— Twisting of Internodes and Leaf -stalks. 



Erect twig of tlie large-flowered Rock Rose (Helianthemum grandiflorum). * Procumbent twig of the same plant. 

 8 Erect twig of Diervilla Canadensis. * Twig of the same plant, bent downwards. 



their neighbours. It naturally very frequently happens that these alterations 

 are also combined in many ways. 



The first instance, the twisting of the internodes, may be observed in hazels 

 beeches, and hornbeams, and especially in trees, shrubs, lianes, and bushes with 

 decussating short-stalked leaves, as for example in Gornus and Thunhergia, in 

 Lonicera and Diervilla, in Androscemum and Hypericum, in Thymus and Vinca, 

 Coriaria myrtifolia, Gentiana asclepiadea, and innumerable others. Fig. 107 ^ 

 represents an erect twig of Diervilla Ganadensis. As soon as such a twig 

 develops no longer upwards, but horizontally, a twisting of 90° takes place in each 

 internode, and the consequence is that the surfaces of all the pairs of leaves take 

 up the same position towards the sun, as shown in fig. 107 ^ The leaves are 

 now no longer arranged in four, but in two, rows. 



Very often twisting of the petioles goes hand in hand with that of the inter- 



