732 RESISTANCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, ANIJ BENDING. 



usually in the shape of tubes of hard bast, or as collenchyinatous strands at the 

 circumference of the stem. In the common Reed (Phragmites communis) tliis tube 

 is quite uninterrupted and intact (fig. 180*); whilst in the Sugar-cane (Saccharum 

 offi,cinarum., fig. 180'') air-canals and vascular bundles are embedded in it. Much 

 less frequently the sti-engthening is produced by Imndles of bast which lie close 

 under the epidermis of the stem and are not fused into a tube, as, for example, in 

 the Hard Rush (Juncus glaucus), the transverse section of whose stem is shown 

 in fig. 180*. This rush is also characterized by the insertion of large air-spaces 

 between the accessory strands which form the outer circle. Some of the erect stems 

 here cited which resLst bending are hollow within, whilst others are filled with a 

 loose pith. In the diagrammatic figures the central cavity has been marked oflF by 

 a circular line. 



We should naturally expect to find that stems which are not able to rise from 

 tlie ground without external support (including those numerous forms which are 

 comprehended under lianes), would exhibit a structure diflferent from that of erect 

 stems. In climbing plants the young shoots alone require to resist bending; stems 

 which have found a support can dispense with this property, and consequentlj^ with 

 contrivances designed for this purpose. On the other hand, these plants, especially 

 when perennial and lignified, must be protected against strains which are unavoid- 

 able in consequence of alterations occurring in their supports. Rocky walls and old 

 battlements overgrown with climbing plants, of course, do not alter sufficiently to 

 materially affect the stems attached to them ; but it is otherwise where the climber 

 is supported by a thickening stem. This class of support continues to grow, its 

 stem increases in volume, the extent of the boughs and branches differs from year 

 to year, and displacements and alterations in position occur which cannot but influ- 

 ence the plants climbing over them. Supjjose a twining plant has embraced and 

 twined around the stem of a young tree or the branch of a young shrub: j-ears pass 

 by and the stem of the tree has meanwhile increased a hundredfold in diameter, 

 and the entwined branch of the shrub has been shifted about a metre; this 

 cannot be without eflFect on the twining stem, and it requires no further explanation 

 to see that it will exert a jduII and lateral pressure. Perennial twining plants must 

 therefore be so organized that their stem will bear tension and lateral pressure 

 without injury, in other words, that their skin must be constructed to resist tension 

 and compression. Resistance to strain is obtained in twining and interweaving stems 

 in very different ways; in many cases, such as in the Rotang or Climbing Palm, by 

 ample depositions of hard bast in the vascular bundles lying next to the axis of the 

 stem; in other cases, e.g. in Tarn/iis and Dioscorea, by a considerable thickening of 

 the cells of the pith, and in others, again, e.g. in many species of Pepper, by the 

 development of a ring of mechanical cells within the peripheral circle of vascular 

 bundles. It is, of course, an advantage to the twining stem which requires protec- 

 tion against strain if the tissues lying next its centre possess a corresponding firm- 

 ness. Thus we find unmistakable diflPerences between these and erect stems; corre- 

 lated with this is the fact that the pith, or the medullary cavity, in twining stems 



