476 MECHANISMS FOR CONVEYANCE TO AND FRO. 



tioned by a layer of tissue in which a very vigorous formation of new cells is 

 carried on, termed the caTnbium (fig. 125^). From this cambium, which appears as 

 a ring in the circular cross section of erect stems, cells develop which on one side 

 abut upon the wood already present in the interior, and on the other the existing 

 bast portion of the vascular bundle to the exterior. In this way both portions, and 

 in fact the whole stem, increase in dimensions; and in the wood, in particular, arise 

 the annual rings which are visible in a cross section. The cambium ring also 

 stretches; it becomes larger and larger, but always retains the same position and 

 relation to the wood and bast of the vascular bundle, and keeps its ring-like form 

 although the trunk in question may have become ever so old and thick, and may 

 exhibit hundreds of annual rings. Here, therefore, the soft bast lies outside the 

 cambium ring, and is screened towards the exterior by various tissues, by hard bast 

 and corky tissue among others, and the latter may undergo considerable develop- 

 ment in trunks of many years' growth; while the hard bast, on the contrary, 

 diminishes in older trunks, because it is no longer required as a protection against 

 bending. Accordingly the soft bast is situated fairly near the surface. Since a 

 strong external lateral pressure is not to be feared in them, this position cannot be 

 characterized as unfavourable. The cork and other external portions of the cortex 

 comprehended under the term bark afford a sufficient protection against small 

 pressures in old stems. In lianes it is very different, especially in those which 

 make use of erect stems as supports. Apparatus for increasing the bearing capacity 

 and elasticity in lianes would be superfluous, these tasks being performed by the 

 support; on the other hand, a protection against lateral pressure is urgently 

 required, for if the support up which the lianes climb, to which they are attached 

 by adventitious roots, or which they encircle and entwine, increases in thickness, as 

 is usually the case, then a lateral pressure on the adherent liane coils is unavoid- 

 able. And when, as a result of such pressure, the sieve-tubes and bast parenchyma 

 become squashed over considerable distances, they are obviously unable to perform 

 their functions satisfactorily, and nutrition will certainly be impaired. Lianes are 

 protected by the most varied contrivances against this source of injury, and some of 

 the most striking will be here briefly indicated. 



In Rhynchosia phaseoloides, the young, green, twining stem is circular in cross 

 section, and exhibits a structure which does not differ materially from that of 

 young normal stems. In the centre is a pith, round which the vascular bundles 

 form a ring — first wood, then soft bast, further out hard bast, then a layer of 

 green cells, and, finally, the epidermis, which envelops the whole. It might be 

 expected that in the second year, the newly-formed cells and tubes would deposit 

 wood on wood and soft bast on soft bast, and that the cylindrical stem would 

 increase regularly in circumference without altering its shape. But, strangely 

 enough, this does not happen. New cambiums arise at two points near the 

 periphery of the stem, below the green cells, by which the formation of wood 

 proceeds in the direction of the first year's vascular bundle ring (i.e. inside), 

 and soft bast accompanied by hard bast on the opposite side (i.e. outside). At 



