172 



MECHANICAL SYSTEM 



the main girders. In Cyperus vegetus (Fig. 57 b) the disposition of the 

 mechanical strands is very similar. Here again the haulm contains 

 a number of subepidermal I-girders arranged in a fairly regular 

 circular series. In this case, however, the two Hanges of each girder 

 are so widely separated, that the mestome-strand no longer suffices 

 to till the whole of the intervening space, but has to be supplemented 

 by parenchymatous cells interpolated between the outer Mange and 



B 



Fig. 57. 



Composite peripheral girders. A. Half of a T.S. through the haulm of Trie/to- 

 phovum germanicum ; the web of each girder consists of a mestome-strand. x 100. 

 B. Part of a T.S. through the haulm of Cyperus reyetus ; the web of each girder 

 consists of a mestome-strand (half-encircled by the semi lunar inner flange) together 

 with some parenchyma (which separates the mestome-bundle from the outer flange). 

 X6(i. 



the mestome-strand, which itself adheres closely to the inner flange. 

 The parenchymatous cells employed for this purpose are specialised 

 photosynthetic elements (palisade-cells) ; the whole girder thus assumes 

 the form shown in the adjoining figure (Fig. 58). 



A much more complicated and elegant type of construction is met 



'with in the haulms of Juncus glaucus. Here again there is a ring of 



regularly spaced main girders, the outer Manges of which consist of 



stout fibrous strands (Fig. 5 9 a), while the inner Manges are composed 



of sickle-shaped fibrous sheaths embracing the large mestome-bundles 



