MOVEMENTS OF GROWTH. 



365 



mined by the magnitude of the osmotic pressure [Turgorkraft], 

 and by the amount of resistance offered by the stretched cell 

 layers (protoplasm, cell-wall). This resistance depends, among 

 other things, upon the extensibility of the stretched cell layers, 

 so that special interest attaches to the exact determination of this 

 extensibility. 



We use for investigation, e.g., an epicotyl of Phaseolus, 30 mm. 

 in length, which has been divided into partial zones 5 mm. in 

 length. We determine its rate of growth, and then plasmolyse it. 

 The limp stem is carefully laid upon a sheet of cork, and covered 

 at its upper end with a small sheet of cork, the two sheets being 

 then clamped together by means of a binding screw. To the 

 older end of the stem we fasten a piece of thread with a loop. 

 This is pulled till the desired extension of the object is obtained, 

 and then fixed to the cork by means of a needle. The stem is only 

 stretched till it is artificially brought back to the length which it 

 possessed before being plasmolysed. By means of a millimetre 

 scale we determine the amount of extension of the individual 

 partial zones, and express the results in terms of the original 

 equal lengths of the zones (5 mm.). It is thus proved that the 

 extensibility of the tissue in the younger regions of the structure 

 is considerably greater than in the older ones. A relation is there- 

 fore established between the rate of growth, the amount of turgor- 

 extension, and the extensibility of the tissue in the different 

 partial zones. 1 Wortmarm, e.g., has, in experiments with a young 

 bean epicotyl, which he had divided into six zones originally 

 5 mm. each in length, determined the following values for the 

 growth of the partial zones in twenty-four hours, for the shorten- 

 ing in salt solution, and for the increase in length by stretching. 



