RESISTANCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, AND BENDING. 



733 



is very much reduced, and that hollow twining stems, e.gr. that of Thunbergia lauH- 

 folia {(•/. tig. 128', p. 477) are very rare. Perennial twining stems are usually 

 protected from lateral pressure by a layer of collcuchynia surrouudiug the conduct- 

 ing tissues like a mantle. Sometimes the collenchyma is also connected with bundles 

 of bast, and there is no doubt that the same mechanical cells which strengthen the 

 young twining stem protect it later on against lateral pressure. 



Perennial climbei-s which have clambered up growing woody plants are exposed 

 to the same dangers as described in the case of twining and interweaving plants, 

 but in them tendrils as a rule afford a protection against tearing, and tissues pro- 

 viding a resistance to strain are absent from the stems themselves. In such plants 

 it is the tendrils especially wliich are constructed to resist 

 tension, as, for example, in tlie Atragene {Atragene 

 alpina, the stem of which is shown in cross section in 

 fig. 181). Tendrils, therefore, are evidently of complex 

 structure. Firet, they must have a great capacity of 

 resisting strain, but since they also have other functions 

 to perform, and since these functions are different before 

 and after the attachment to the sujjport, very remarkable 

 alterations in their inner structure must occur during 

 development. At first they are required to resist flexion, 

 for which purpose mechanical tissue is developed round 

 the periphery; later on they have to resist tension which 

 renders it necessary that mechanical tissue should be 

 developed nearer the axis. An abundant development 

 of mechanical tissue is also required on the convex side 

 of the tendril bending round the support so as to increase 

 the resistance to strain at that part, as also to prevent its unrolling from the sup- 

 port; such a development is actually to be seen in all tendrils. 



Older lignified stems of climbing and twining plants often exhibit a longi- 

 tudinal splitting in the wood. Before they obtain their split appearance the 

 narrow vascular bundles, which consist for the main part of wood, are isolated by 

 a loose, wide-meshed tissue, and there is no central pith. In transverse section 

 the narrow vascular bundles of such a stem resemble the spokes of a wheel, the 

 w^eakly-developed mechanical tissue, which had served to pi-otect against bending 

 in the one-year-old stem, together with the cork (periderm), forming to some 

 extent the rim of the wheel (c/. fig. 181). 



When lateral pressure is brought to bear on these old stems, the cork and 

 hard bast become ruptured at the places acted on, but only above the dead, large- 

 meshed tissue; above the narrow vascular bundles they remain uninjured. Tlie 

 loose, dead tissue also ruptures and crumbles, and falls out of the grooves 

 between the vascular bundles. These bundles, which now resemble plates or 

 lamelljB of wood, lie above one another like the leaves of a book on the side where 

 the pressure is felt. The wood looks as if it had been divided or split longi- 



Fig. 181.— Tmnsveree section of the 

 climbing stem of the Atragene 

 (Atragene alpina). Tlie tissues 

 are represented in tlie following 

 way :— Soft bast, entirely black ; 

 wood, larger and smaller white 

 dots on a blaclc ground ; the me- 

 chanical tissues, obliquelyshaded; 

 the cork (periderjn), stratified ; 

 the loose reticular tissue, white 

 with dark reticulations. 



