402 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY 



SECTION 88. The Forms of Physical Energy used by Plants. 



OSMOTIC ENERGY. This special form of volume energy comes into play 

 whenever a soluble substance is unevenly distributed. The diffusion thereby 

 produced takes place in the same way as when different gases are mixed 

 together, and if the particles of the dissolved solid or gas are unable to pass 

 through a separating membrane, they bring to bear a pressure upon it which 

 is dependent upon the number of the molecules in unit volume and upon 

 their average kinetic energy. The latter, again, is constant for any large 

 number of molecules at a given temperature and is equal to half the product 

 of the mass of each molecule multiplied by the square of its average velocity. 

 A rise of temperature which increases the velocity of movement of the 

 molecules causes a slight rise of osmotic pressure, and also of the pressure of 

 a gas kept at constant volume. 



A purely physical diffusion movement must take place whenever any 

 difference of concentration is produced in parts separated by permeable 

 partition-walls. If, however, the walls are semipermeable a permanent 

 osmotic pressure can be maintained, such as is commonly used in plants 

 for various mechanical purposes. A naked cell or gymnoplast would be 

 indefinitely stretched or burst by a high internal osmotic pressure, whereas 

 in dermatoplasts covered by a cell-wall a comparatively high pressure is 

 often required to render them fully distended and active. When death or 

 plasmolysis allows the cell-wall to contract, the potential energy latent in 

 it when stretched is manifested. Sudden decreases of turgor produced by 

 a physiological reaction are responsible for the rapid movements of the 

 stamens of Cynareae and of the leaves of Mimosa, which can be repeated 

 as soon as the original turgor has been restored. The pulsation of certain 

 vacuoles is, in some cases at least, produced by automatic variations of turgor. 



In order to maintain the turgor in a growing and enlarging cell, 

 a regulated production of osmotically active materials is necessary. During 

 plastic growth the mechanical work involved in the stretching of the cell- 

 wall is carried out by the previously accumulated osmotic energy. If, 

 however, the growing organ encounters a resistance, the tension in the 

 apposed cell-wall gradually decreases until nearly the whole of the osmotic 

 pressure is acting against the resistance. 



By a similar counteraction of active and passive tissues, tensions and 

 pressures are produced which when released may lead to sudden move- 

 ments, as during the dehiscence of the fruits of Impatiens or of Momordica. 

 In these cases the potential energy is stored up by a definite physiological 

 activity, whereas a purely physical action on a given mechanism is involved 

 when the dry valves of the fruits of Leguminosae twist on drying 

 and untwist on moistening, or when leaves droop for want of water and 



