MECHANISMS FOIl COXVKYANCE TiJ AND FRO. 



477 



tlie end of the second yejvr the stem is no longer circular, us at the first; it 

 has added two rings, as it were, and now appears elliptical in cross section; 

 aiul since new portions are added in this way repeatedly from year to year, 

 anil new rings are always becoming annexed to those already existing, the 

 stem gradually becomes ribbon-like, and fxliihits a cross section like that shown 

 in fig. 128 -. The soft bast has thus received the most protected position 

 imaginable, and lateral pressure is unable to interfere with its functions. When 

 tlie supporting stem round which the Rhynchosia has twined grows enormouslj' 

 in thickness, the liane becomes stretched, and experiences a lateral strain, but 

 tlie sap can, nevertheless, continue its journeys unhindered in the soft bast. 



In the liane Thunbergia laurifolia, a cross section of whose stem is represented 

 in fig. 128 \ the protection is obtained in quite a different way. Here the gi-een 



Fig. 12S.— Transverse sections of Liane Stems. 



i Thunbergia lauri/otia. ^ lihynchosia phaseoloUies. ^ Tecoma radicans ; x 30. Diagramm.atic. The v.irious tissues are 

 indicated in the following manner; Soft bast, entirely black; wood, larger and smaller white dots on a black ground; 

 bard bast and other mechanical tissues, obliquely shaded ; cork (periderm), sliurt Hues ; pith, reticulated. 



stem is hollow, and the cavity is surrounded by an enormous pith. In the 

 vascular bundle ring which surrounds the pith, the wood and hard bast are 

 not arranged from the first in successive concentric circles, as is usually the case, 

 but are placed side by side. The cambium continues to form soft bast in some 

 places, and wood in others, towards the interior. In consequence of this, the 

 bundles of soft bast appear to be walled in by the wood (" bast-islands "), and 

 are consequently well protected against pressure. The protection is increased 

 by the fact that this liane is hollow in the centre, and can " give ", an un- 

 common feature in twining plants. 



Sometimes the delicate soft bast is protected against compression by its 

 position in niches and grooves at the periphery of the hard wood; this is to be 

 seen especially in several twining Asclepiadeae and Apocynacere. One of the 

 most remarkable protective arrangements is found in the climbing Tecoma 

 radicans, which adheres to its substratum by tufts of aerial roots, and whose 

 leafless branches are depicted in fig. 120. A cross section of the stem is shown 



