THE MAINTENANCE OF BODILY FORM. 1 23 



In stems of trees rigidity is secured not by the distribution 

 of the mechanical tissues, but by their massiveness. In 

 them the chief mechanical tissues belong to the wood, which 

 forms a solid column occupying the center of the body. 

 Aquatic plants, which are supported by the medium in which 

 they live, are usually without mechanical tissues. 



159. Summary. — Bodily form is maintained by naked 

 protoplasm chiefly by unknown forces inherent in the living 

 substance. In larger plants it is maintained partly by turgor, 

 which develops opposing strains in masses of cells, and partly 

 by the mechanical stiffness of the cell-walls formed by the 

 protoplasm. These rigid parts are either massed or definitely 

 placed in the plant body, so as to carry its weight and meet 

 the strains due to winds, water-currents, etc. 



