210 TEXTBOOK OF BOTANY 



By means of the facts and inferences already encountered and the 

 brief summary of several examples in this chapter, the reader is offered 

 an opportunity ( 1 ) to check his own observations of the kinds of plant 

 behavior that are consequences of the conditions underlying osmosis and 

 cell turgor, and (2) to evaluate conflicting interpretations of the ob- 

 served behavior. Movements in some plants differ from those in other 

 plants because the plant structures are different. A consideration of how 

 these differences in structure originated must of course be postponed to 

 the chapters on heritable variations and evolution. They are, however, 

 the consequences of processes to which no one would ascribe intelligence 

 and purpose unless he is accustomed to ascribe these human attributes 

 to all physical and chemical processes in the universe. 



Rigidity of plants. The rigidity of all herbaceous parts of a plant is de- 

 pendent upon the turgidity and mutual pressure of cells described above. 

 During dry weather leaves, young stems, flowers, and even fruits may 

 wilt during the day and become firm at night or following rains, as the 

 result of changes in the turgor of the individual cells. Spray systems 

 are now commonly installed over market stands to prevent the loss of 

 turgor in the cells of leafy vegetables on display. The rigidity of woody 

 tissues is dependent not upon osmosis and turgor but upon the thickness 

 of the walls of the wood cells. 



Physiological drought. Owing to the relatively high concentration of 

 water in the vacuoles of their cells, many fresh-water plants cannot grow 

 in the ocean or in salt lakes and salt marshes where the concentration 

 of water is relatively low. The fact that water plants may wilt in such 

 habitats can be demonstrated by placing a plant in a vessel of salt water. 

 Similarly, most land plants do not grow on salt plains (Fig. 71). Man 

 sometimes makes miniature salt plains by adding salt to tlie soil to kill 

 weeds. One of the difficult problems encountered in applying irrigation 

 water to soils of arid regions is the necessity of preventing the accumu- 

 lation of inorganic salts in the soil surface as the added water evaporates. 

 The preservation of food in water of low concentration ( brine ) is a very 

 old custom. Sugar and salt are used as the means of diluting the water 

 to the point of physiological drought for the bacteria and molds that are 

 responsible for the spoiling of food. 



Vinegar is also used to prevent the growth of these organisms. It is 

 primarily a solution of acetic acid in water. This acid lowers the con- 

 centration of water in vinegar and also coagulates the protoplasm of 



