40 COBN CBOPS 



ever, when the weather is very hot or when soil moisture 

 is low, the cells lose water enough so they are no longer 

 turgid, and the leaf then becomes limp and rolls up. In 

 the same way, all cells of the plant may be more or less 

 turgid, aiding in giving rigidity to the plant body. 



30. Tension. — ■ If a section a few inches long of the 

 stem of green corn be taken and the outer peripheral tissue 

 be removed from the pith, the pith will at once expand in 

 length and some force will be required to restore it to 

 normal length. It will thus be seen that there is a natu- 

 ral tension at all times between the outer cortex and the 

 pith. This tension adds to the rigidity of the stem. 



31. Mechanical tissue. — The supporting framework 

 is made up of woody and fibrous tissues in the outer part 



and the nodes of the stem and in the 

 midribs and veins of the leaves. These 

 are mostly comprised of fibers (scleren- 

 chyma or bast) of great tensile strength. 

 Quoting from Strasburger, " the sus- 

 taining strength' of sclerenchymatous 

 fibers is, within the limits of their elas- 

 ^rng're^Tswft ^i^it^' in general equal to the best 

 bending when the wrought iron. Or hammered steel." 



™f outewTof ^^^ ^^^'^ ^'^ b^^^'i together, giving 



the stem, as in corn, a strong elastic body. 



One side must be The location of the framework on the 



shortened and the . , 



other stretched. outside, rather than m the center, of 



the stem adds to the rigidity. For 



example, if an elastic rod be bent (Fig. 15), the inner side 



is shortened and the outer lengthened. If a supporting 



skeleton be placed in the center of this rod, then the rod 



is flexible and considerable bending would be possible 



without much resistance from the center ; but if the sup- 



