CYLINDRICAL INFLEXIBLE ORGANS 



173 



which lie on the same radii. The space between the two flanges of each 

 girder is occupied by the following tissues in centrifugal order; first, 

 a large mestome-strand, next the small outer fibrous sheath, then thin- 

 walled parenchyma, then a wide air-passage, beyond which parenchy- 

 matous tissue once more appears. The web here exhibits two un- 

 expected features, the more remarkable of these being the occurrence 

 of an air-passage, which must weaken fj 



the radial connection between the two 

 flanges, notwithstanding the presence 

 of a loose " stellate tissue " in its 

 interior. It must not be forgotten, 

 however (cf. above, Section III., A.), 

 that there is no need for a continuous 

 web, provided that the tangential con- 

 nection between the adjacent girders, 

 or groups of girders, is sufficiently strong. 

 From this point of view T it must be 

 admitted that the inclusion of the large "' 

 air-passages in the web of the girders 

 is not only free from objection, but 

 actuallv advantageous, inasmuch as the 

 passages interfere in this way as little 



as possible With the tangential COn- A single composite peripheral girder from 



. r> 1 l l rpi, the hau]m <){ Ch/ii r a. <,- 'Ht ft. b, outer flange 



tmUlty Of the mechanical System. llie of bast; & inner flange ; <7, mestome-bundle ; 



, ,. ,. ., ,- ., parenchymatous bundle-sheath. 



second outstanding peculiarity of the 



girders in Juncus glaums is the presence of small fibrous sheaths 

 on the outer sides of the mestome-strands. It is not at once 

 obvious why plastic materials are expended in the production 

 of these outer sheaths, instead of being utilised to strengthen 

 the inner flanges. It is necessary, however, to recollect that fibrous 

 strands may have a strictly local mechanical value; in the present 

 instance, for example, the outer sheath serves to protect the delicate 

 protein-conducting portion of the mestome-strand. 



The somewhat wide intervals between the successive main girders 

 of Juncus glaums are occupied by symmetrically arranged subsidiary 

 girders, with webs consisting of mestome-strands and nothing else. 

 Each group of subsidiary girders, which " forms a veritable phalanx 

 of four to six members," serves to give the necessary rigidity to the 

 comparatively wide partitions that intervene between the main girders. 



(c) Third system. Subcortical Jibro- vascular strands. 



In the types belonging to the third system the fibrous bundles are, as 

 it were, repelled from the surface, and thus forced into a more central 



Fig. 58. 



