RESISTAXCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, AND RENDING. 735 



soft bast against lateral pressure. The ribbon-shaped stem, however, can do very 

 well without this, for the pressure along its edge is scarcely worth considering, 

 and the soft bust is excellently protected against pressure on the broad side by 

 the wood, which is broken up into a number of detached masses with the soft bast 

 between. 



There is no doubt that the spiral torsion of ribbon-like lianes (which is plainly 

 shown in the illustration of lihynchosia phaseoloides, tig. 127, on p. 475) increases the 

 resistance to strain, a matter of some importance in all cases where growing trees 

 or shrubs serve as supports, and where straining of the lianes clinging to them 

 is unavoidable. 



The undulations of ribbon-shaped liane stems in tropical forests may also be 

 regarded as a protection for the sap-conducting tissues against strain. They occur 

 in many bauhinias and in 

 the peculiar species of C'aii- 

 lotretus known as monkey- 

 ladders. The central part 

 of the ribbon-shaped stem 

 is alone strongly undulated, 

 as may be seen in the por- 

 tions of a Bauhinia repre- 

 sented in fig. 182; the two 

 edges are much less curved 

 and are often quite straight, 

 forming a framework for 

 the sinuous middle part. 

 In the case of a longitudinal 

 tension, at first only the 



frame is aflfected, the tissues in the centre can still uninterruptedly conduct the sap 

 to and from the branches which arise from its broad surface. 



Stems of water-plants as well as those embedded in the ground, and the stem- 

 structures which lie on the surface of the ground, have, like climbing plants, little 

 need for resisting flexion, but, on the other hand, require a greater retdstance to 

 pressure and strain. The soil or the surrounding water forms the immediate sup- 

 port for all these stems, and the arrangement of tissues suited to erect aerial stems 

 would be useless here. 



As a matter of fact they do not possess the peripheral strands of hard bast 

 and coUenchyma so characteristic of erect stem-sti'uctures; the vascular bundles 

 are placed together near the centre of the stem, as is most advantageous for organs 

 which have to resist strain, and the bast strands belonging to these bundles are 

 relatively far removed from the circumference of the stem. The central pith is 

 much reduced and is often completely absent (c/. the diagrammatic sections of a 

 runner of the Garden Strawberry, Fragaria grandifiora, and of a hydrophyte, 

 Myriophyllum spicatum, in the above figure). 



Fig. 183. 



I Transverse section of a runner of the Garden Strawberry (Fragaria grandijlora) 

 wliich lies on the ground. 2 Transverse section of the stem of the Water 

 Milfoil {ityriophyllum spicatum). In these diagrammatic figures the mechani* 

 cal tissue is represented grey, and the vascular bundles black with white 

 spots. 



