THE MAINTENANCE OF BODILY FORM. 149 



diameter and length slightly, and gains, at the same time, a 

 condition of rigidity greater than in its unstretched condi- 

 tion. In a similar way turgid cells are more rigid than those 

 which are flaccid. The union of turgid cells produces a 

 member more rigid than one in which the cells are not turgid. 

 An illustration of this is to be seen in the condition of a 

 wilted, as compared with a fresh, leaf. The turgor of thin- 

 walled cells may play an important part in maintaining the 

 form and position of the parts of a plant. 



189. Tissue tensions. —But turgor can affect only those cells 

 whose walls are thin and extensible. Those whose walls have 

 become thick and rigid are not stretched by this force. In 

 the larger plants, however, where both thick-walled and thin- 

 walled tissues exist, it is possible that a mass of thin-walled 

 cells may, by the united tension of its component cells, 

 stretch those tissues which are not themselves turgid. Such 

 strains in the younger regions, particularly, play an important 

 role in maintaining the form of these parts. But the tensions 

 in the older parts are generally due to the unequal growth of 

 different tissues. (See ^[ 259.) 



190. Mechanical rigidity. — The rigidity of the cell-wall 

 itself must be relied upon by all the larger plants. Certain 

 tissues are specialized by having their cell-walls greatly 

 thickened, and such tissue regions constitute a sort of frame- 

 work or skeleton, which is filled out by the more delicate 

 tissues. These mechanical tissues are so distributed within 

 the body as to afford frequently the maximum resistance to 

 bending and breaking strains. In the accompanying dia- 

 grams the position of the mechanical tissues is indicated in 

 transverse sections of a number of different stems (fig. 170). 

 It will be seen that they illustrate well-known mechanical 

 principles in their distribution. The hollow column (E) and 

 the I-beam [A, B, C), two of the most rigid mechanical con- 

 structions, are frequently imitated in plants. 



