30 RELATIONS OF PLANTS TO MECHANICAL FORCES 



sure, alterations in the rapidity of growth, and the differentiation 

 of the tissues. The reactions may really go so far as to deter- 

 mine the development or suppression of organs, as illustrated by 

 the growth of secondary roots from the convex surface of main 

 roots alone. 



Mechanical stimuli of the above character may act from with- 

 out, although the greater number originate within the body of the 

 plant. Any organism consisting of more than one cell, has the 

 development of its tissues or cell-masses affected by their mutual 

 mechanical relations. In large plants such as trees, in which the 

 living tissue is held firmly by a cylinder of dead wood and en- 

 closed by a sheath of corky tissue the compression and its effects 

 are most marked. The increase in weight of a fruit will cause 

 morphological changes in the stem supporting it, and the multitu- 

 dinous stresses set up by the stretching, bending and compressing 

 action of neighboring tissues of unequal growth find responses 

 in the tissues affected. The presence or absence of the action 

 of some of these mechanical forces of external or internal origin 

 may be accountable for the presence or absence of certain 

 kinds of tissues in some plants. Thus, for instance, elongated 

 bast fibres may be produced in certain plants by the action of 

 external mechanical bending forces, although usually absent. 

 The relative weight and density of the medium in which the 

 organism lives are of course trophic conditions which may not 

 be greatly varied without detriment to the normal processes of 

 the organism. 



44. Changes in Tendrils due to Pressure. The process of 

 curvature in tendrils presses the inner flank against the object 

 around which they are coiled, which induces altered develop- 

 ment of the tissues, consisting in the increase of the wood ring, 

 the tangential division of the hypodermal layers, with an increase 

 in the thickness of the walls. Cut cross and longitudinal sec- 

 tions of a portion of a mature tendril which has been tightly 

 coiled around a support for several days. Note the structure of 

 the hypodermal layers, cortex, and fibrovascular ring. Compare 



