188 MECHANICAL SYSTEM 



organs to be the same as in inflexible structures, and the subject need 

 not be pursued any further. 



Greater interest attaches to those organs which behave as inex- 

 tensible or as longitudinally incompressible structures according to 

 circumstances. This complex condition is exemplified by the stilt- or 

 buttress-roots which occur in the genus Pandanus, in Iriartea exorrhiza. 

 and in Bhizophora Mangle and other Ehizophohaceae ; further, by the 

 whorled adventitious roots which issue from the lower nodes of the 

 stem of the Indian Corn, and which are physiologically equivalent to 

 the aforesaid buttress-roots. 



Warming 103 has shown that the stilt-roots of Bhizopkora Man;//' 

 are very stem-like in structure : the centre of each root is occupied by 

 an extensive pith, which is surrounded by a number of alternating 

 groups of leptome and hadrome that collectively form a hollow poly- 

 arch vascular cylinder or stele. The water-conducting portions of the 

 vascular cylinder are accompanied, on the medullary side, by thick- 

 walled mechanical cells, which are clearly arranged so as to render the 

 root inflexible [and hence longitudinally incompressible]. At a later 

 stage the organ is still further strengthened by the development of 

 a secondary woody cylinder containing numerous fibrous elements. The 

 stilt roots of Bhizopkora mucronata and Bruguiera eriopetcda are similarly 

 constructed. Those of Zea Mays, however, require more detailed con- 

 sideration. The subterranean root-system of the Indian Corn does not 

 provide adequate support for the rapidly growing shoot. Hence, 

 adventitious roots are produced from the lower part of the aerial stem, 

 where they form a very regular whorl at each node. These roots do 

 not, however, grow straight downwards, but follow a more or less 

 oblique course, all the members of a whorl forming about the same 

 specific angle with the vertical (Fig. 66, V, V); they enter the soil 

 at a distance from the stem which varies according to the height of 

 their point of origin, and thereupon produce numerous rootlets, which 

 anchor them firmly in the ground. 



Evidently when a Maize-stem bends before the wind, the stilt- 

 mots on the windward side are stretched, while those which extend to 

 the leeward are longitudinally compressed. Each root therefore has to 

 behave, at different moments of its existence, now as an inextensible, 

 and now as an inflexible organ ; the structure of the root corresponds 

 exactly to this alternation of mechanical requirements. The fibro- 

 vascular cylinder, instead of being solid as in most roots, encloses 

 a wide core of pith, so that the mestome and the accompanying stereides 

 together form a hollow cylinder (Fig. 65 b). The latter has its 

 mestome elements arranged in the manner characteristic of normal 

 roots. The most conspicuous components are certain very wide vessels 



