GROWTH 



115 



causing the long arm of the index to move up over the 

 graduated scale. If the salt-solution is now removed, and 

 the jar filled with tap water, or better, distilled water, 

 the water will enter the cells of the stem by osmosis, in- 

 creasing the internal osmotic pressure and turgor of each 

 cell. As a result the stem as a whole will elongate or grow 

 in length, thereby causing the index to move down over 

 the scale. 



Fig. 73.- — Experiment to demonstrate the relation between osmosis 

 and growth in length. /, jar containing water, and subsequently salt- 

 solution; p, portion of leaf-stalk of Rhubarb; /, index-lever (portion 

 omitted at b); S, scale. Explanation in text. 



118. Differential Growth. — Not all the tissues of a 

 stem or other part grow at the same rate.^ On this ac- 

 count, and since adjacent tissues are closely united, those 

 which elongate or grow more slowly are stretched by those 

 which grow more rapidly. As a result either a state of 

 tension exists, or the organ is distorted, or both. When 

 one epidermis of a leaf grows more rapidly than the other, 

 distortion results, and the leaf becomes ''crisped" or 

 crinkled. This is normally the case in some plants, but 



* The student should endeavor to reason out an explanation for this. 



