CHAPTER XXIII 

 PLANT BEHAVIOR RELATED TO OSMOSIS 



Osmosis and the turgidity of cells are primary causal factors in certain 

 kinds of plant behavior that have attracted the attention of almost every- 

 one. We have already seen that when the concentration of water sur- 

 rounding a plant cell is higher than its concentration within the vacuole, 

 the water diffuses into the cell, thus causing an increase in cell turgor 

 and also in cell size if the cell wall is extensible. The converse occurs 

 when the higher concentration of water is within the vacuole: water 

 diffuses out of the cell, cell turgor decreases, the cell may become 

 smaller, and in extreme cases plasmolysis, wilting, and death may result. 

 When all the cells of some part of a plant (leaf, flower, stem, etc.) in- 

 crease in size they press against each other, with the result that the 

 whole plant structure becomes firm and rigid. On the contrary, when all 

 the cells of a plant shrink in size their pressure against each other di- 

 minishes and the plant becomes flaccid, or wilts. If the cells on one side 

 of an organ shrink or swell more than those on the opposite side, curva- 

 tures or movements result. 



Movements and growth curvatures. In growing regions increase in cell 

 size is dependent both upon the entrance of water and upon an accom- 

 panying growth in area of cell walls. Increase in area of a cell wall, 

 therefore, may be the result of stretching or of growth. The former may 

 be reversible, the latter is non-reversible. Likewise, movements and 

 curvatures that are dependent solelv upon turgor and the elasticity of 

 cell walls may be reversible. Growth curvatures may be permanent or 

 temporary. For example, the growth curvature of a stem toward the more 

 intense light during the dav mav be annulled by the greater growth of 

 the cells on the concave side of the stem tip during the night. 



Superficially these movements and curvatures of plant organs may 

 appear to be intelligent acts of the plant. Speculations as to their advan- 

 tages or disadvantages are not uncommon. Perhaps the reader will dis- 

 cover that they are all consequences of similar fundamental conditions 

 and processes, regardless of whether they have "survival value." 



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