LIANE-STEMS 



()95 



their inextensible character becomes less pronounced, just as inflexibly 

 constructed trunks and branches do not retain the peripheral disposition 

 of their stereome, after secondary thickening has been in progress for 

 some time. In both cases, it is the numerical strength of the scattered 

 mechanical elements that compensates for their failure to congregate 

 at the centre or near the periphery, in accordance with approved 

 mechanical principles. 



The anomalous modes of thickening characteristic of climbing 

 stems, besides leading to the rope-like construction based upon a 

 splitting up of the woody cylinder, also involves another deviation 

 from the normal arrangement of 

 tissues ; for the secondary leptome, 

 instead of occupying a peripheral 

 position, as it does in stems with 

 normal secondary growth, shifts in- 

 wards at any rate to a large extent 

 and becomes wedded in between 

 the various strands or cylinders 

 of secondary wood. The formation 

 of such inter-xylary phloem often 

 depends upon the fact that the cam- 

 bium, besides cutting off secondary 

 phloem on its outer side, also gives 

 rise on its inner side to isolated 

 strands of sieve-tubes, accompanied 

 by parenchymatous tissue ; these 

 intra-cambial masses of phloem 

 (such as occur in Dicclla, Combretum, 

 Mueuna, Entada, etc.) thus be- 

 come embedded in secondary wood. 

 According to Leisering, however, the relation of such internal phloem- 

 strands to the cambium is indirect, since they become secondarily 

 differentiated within masses of tissue resembling unlignified xylem- 

 parenchyma. In the genus Strychnos, leptome-strands of extra-cambial 

 origin subsequently become enclosed in the woody cylinder, as a result of 

 special growth-processes. 



That inter-xylary leptome is particularly well protected against 

 mechanical injury is an indisputable fact, and doubtless also an 

 advantageous circumstance, since liane-stems are particularly liable to 

 be subjected to violent flexure, to say nothing of the radial compression 

 produced by the expansion of the living support. It must, on the 

 other hand, be noted that inter-xylary leptome-strands do occur in a 

 number of woody plants which are not climbers, a circumstance which 



Fig. 291. 



Sector of a T.S. through the stem of Gnetum 

 seandi ns. L, 2, 3 successive vascular cylinders ; m, 

 pith ; r, mechanical cylinder situated at the inner 

 boundary of the cortex. Xylem-strands shaded 

 (except for the cavities of the large pitted vessels) ; 

 phloem, cortex and medullary rays shown in 

 white. XS. After De ISarv. 



