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; 7, 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. 1 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 
1 The student should endeavor to reason out an explanation for this. 
