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 these 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 oi" the leaf-surfaces pi'oceeding from 

 the horizontal twigs is carried out, and, indeed, in four difl'erent 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. 1U7. — iwistiii^ iif internodes and Leaf -stalks. 



' Erect twig of the large-Howercii llock Kose (Belinnthemum grandiflomm). * Procumljent twig of the same plant. 

 * Erect twig of Diervitla CaiiademU. * Twig of the same plant, bent downwards. 



their neighbours. It naturally very frequently happens that these alterations 

 are also combined in many way.s. 



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

 beeches, and hornbeam.s, and especially in trees, slirubs, lianes, and bushes with 

 decus.sating short-stalked leaves, as for example in Cornus and TItunbergia, in 

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

 Cormma myrtifolia, Gentiana asdepiadea, and innumerable others. Fig. 107 ' 

 represents an erect twig of Diervilla Canadensis. 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 tJiat of the inter- 



VoL. I. 27 



