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iHi. 





CHAPTER IV 



MECHANICAL PROBLEMS : PROTECTION 



I. Mechanical Problems 



§ I. Mechanical Tissues. § 2. Mechanical Features of the Root 

 System. § 3. Mechanical Features of the Stem. § 4. Mechanical 

 Features of Leaves. § 5. Aquatic Plants. § 6. Climbing Plants. 



The support of the great canopy of foliage with its scaffolding 

 of branches in an ordinary broad-leaved tree can be effected 

 only by a first- class mechanical system. The efficacy of 

 the system is exhibited by the way in which such trees 

 resist violent storms of wind, even in early autumn, when 

 the leaves still offer great resistance to violent gales. 

 When the tree does fall it is usually because it has been 

 uprooted, not because the sub-aerial mechanical system 

 has given way. The excellence of the materials of which 

 the system is constructed is testified to by the uses to which 

 wooden beams are put by man. In lesser plants the same 

 necessities of support exist in varying degrees ; it is in 

 herbaceous plants that the advantageous disposition of 

 mechanical tissues is seen most strikingly. The mechanical 

 tasks of different organs are, of course, different, and the 

 necessities vary with habit and habitat. Our knowledge of 

 the architecture of the plant from this point of view is 

 chiefly due to Schwendener (1874) and Haberlandt. 



§ I. Mechanical Tissues 



Rigidity in stem and root may be due to three different 

 causes : (a) to turgidity of living parenchymatous cells ; 

 (b) to the presence of non-lignified mechanical tissue termed 



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