INCOMPRESSIBLE ORGANS 187 



cortex; these isolated strands are evidently intended to prevent the loose 

 cortical tissue, with its numerous air-passages, from becoming torn, or 

 from being stripped off bodily. 



At this point attention may also be drawn to the inextensible 

 construction of pendulous fruit-stalks and inflorescence-axes. Here 

 the requisite inextensibility is provided for, either by contraction of 

 the fibrous cylinder, or by strengthening of the fibrous sheaths that 

 accompany the internal mestome-strands (Platanus, Sianhopea insignis). 

 Mention may finally be made of . the fact that the pendulous pinnae 

 of the Palm Martinezia Lindcniana are, according to Koop, inextensible 

 in structure, the central region of the mesophyll being traversed by 

 a very large number of stout fibrous strands. There can be no 

 doubt, however, that these deep-seated strands do not, in the first 

 instance, serve to support the comparatively insignificant weight of 

 the pinna : they come into action, principally, when the pinnae are 

 agitated by the wind and thus subjected to much severer tensions. 

 A similar structure has already been described as characteristic of the 

 majority of pinnate Palm -leaves. 



C. STRUCTURE OF INCOMPRESSIBLE ORGANS. 



1. Resistance to radial pressure. 



Every subterranean or submerged organ is subjected to radial 

 compression by the surrounding medium. The cortical parenchyma 

 of such organs usually contains air-passages, and is otherwise loose in 

 texture ; in this event the structure has to be protected against com- 

 pression by means of a firm peripheral shell. Sometimes the epidermis, 

 in conjunction with one or two of the subjacent layers, is able to provide 

 the requisite protection (for example, in the genera Naias and Sagittaria, 

 in Juncus supinus, Alisrna natans and other plants which grow in 

 stagnant or slowly running water). In such organs, on the other 

 hand, as grow in mud or in any saturated soil, the large size of the 

 cortical air-passages necessitates the presence of thick- walled parenchyma 

 or genuine bast-fibres. The roots of certain Cur lees (C. stricta, C. eaes- 

 pitosa, C. vulgaris), and of many GKAMINEAE, are accordingly provided 

 with a tubular sheath of fibres with partially suberised walls. 



2. Resistance to longitudinal compression. 



Every upright axial organ which has to bear the weight of branches 

 and foliage must possess the same kind of rigidity as a pillar or 

 column. It has already been explained, at the beginning of the present 

 chapter, that general mechanical principles render it necessary for the 

 disposition of resistant material in such longitudinally incompressible 



