BILATERAL INFLEXIBLE ORGANS 



179 



A more advanced stage of specialisation is illustrated by those 



leaves (and midribs) in which the mechanical system is unequally 



developed on the two faces, in accordance with an unequal distribution 



of mechanical requirements. In all long, curved, ribbon-shaped leaves, 



the upper side is particularly exposed to tension, while the lower 



side is more liable to he subjected to 



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compression. Evidently, broad sub- 

 epidermal plates of bast are best fitted 

 to ensure a uniform distribution of 

 tensions on the upper side, whereas 

 the lower side can be satisfactorily 

 protected against compression by 

 means of ordinary girders. This form 

 of mechanical system is exemplified 

 by the leaf-midribs of Eriantlms, 

 Saccharum, Zea (Fig. 63 c), Gynerium, 

 etc. Another illustration is provided 

 by the leaf-blades of many Carices ; 

 here the leaf is V- or U-shaped in 

 section, so that the need for inextensi- 

 bility is greatest along the two 

 margins, each of which is accordingly 

 provided with a special fibrous strand. 



Girders of the internal type, 

 characterised by the fact that the 

 outer fibrous sheaths of the mestome- 

 bundles are separated from the 

 epidermis by photosynthetic tissues, 

 are found in certain species of Allium, 

 in the genera Tritonia, Aspidistra, etc. 



Mixed girders occur in the leaf- 

 blades of certain Palms. The larger 

 of the bundles that lie deeply em- 

 bedded in the mesophyll are embraced 

 both internally and externally by 

 fibrous sheaths ; the resulting fibro- 

 vascular strands constitute internal girders, the location of which 

 midway between the two surfaces of the leaf appears at first 

 sight to be unsuitable from the mechanical point of view. In many 

 cases (Livistona chinensis, Chamaerops excelsa), indeed, where the 

 fibrous sheaths of the mestome-strands are feebly developed, their 

 mechanical significance must be mainly local, consisting in the pro- 

 tection of the individual conducting strands. In other instances, on 



Girders in various Monocotyledonous leaves, 

 seeninT.S. A. Pkormiumtenax. Upperand 

 lower flanges of approximately equal size ; 

 web composed of mestome. B. Carex aylva- 

 tica. Upper flange small and tangentially 

 flattened ; lower flange large ; web comprises 

 a mestome-bundle with a fibrous sheath on 

 the hadrome-side, and colourless parenchyma. 

 6'. Zea Mays (mid-rib). Upper flanges de- 

 veloped as tension-flanges. The lower or com- 

 pression-flanges enclose mestonie-bundles. 



