404 TRANSFORMATION OF ENERGY 



Under these circumstances it is but natural that more interest centres round 

 those other forms of energy which may be considered as the immediate causes 

 of movement, simply because they are more clearly and more readily compre- 

 hended. It is PFEFFER'S (1892) special merit that he has most exhaustively 

 catalogued the forces which, apart from respiration, enable the plant to carry 

 on work. The following are the forces which operate independently of 

 chemical energy : 



1. The transformation of potential into kinetic energy which takes place 

 in the balance in tensions, to which may be referred so many of the slinging 

 movements in fruits, stamens, &c., and which manifest themselves in a variety 

 of ways, e.g. by unequal swelling of different wall layers, unequal osmotic 

 pressure of adjacent cells, &c. 



2. By the action of osmotic energy which not only brings about movement of 

 food-stuffs but leads to vigorous pressures and tensions in the plant. The osmotic 

 energy of a substance is quite independent of its chemical energy, and hence 

 cannot be derived from its heat of combustion. An example quoted by 

 PFEFFER will make this clear. If we assume that the osmotic pressure in a 

 cell is due to glucose dissolved in the cell-sap then we are dealing with a 

 substance which not only possesses a high osmotic energy but also a significant 

 amount of chemical energy. Suppose, however, that the glucose be completely 

 respired into oxalic acid, in that oxidation the cell-sap loses the chemical 

 energy set free, but at the same time its osmotic energy is tripled, for highly 

 oxidized bodies possessing little chemical energy may yet exhibit large amounts 

 of osmotic energy. 



3. Quite independently of chemical energy there are various forms of 

 surface energy, such as swelling and surface tension, to which will presently 

 be referred many conspicuous movements in plants. 



4. ' Form energy,' e. g. cohesion, which should also be referred to here. 



5. Finally, we may note the energy of crystallization and secretion, which 

 frequently plays an important part in the growth of the cell- wall. 



The mechanical value of these various forms of energy may often be directly 

 measured in a variety of ways, and are, as we have already said, for that reason, 

 more readily comprehended than chemical energy, whose mechanical equivalent 

 is unknown. We must not forget, however, that although mechanical energy 

 plays an extremely important part in the plant, it is a great mistake to refer 

 all the best known cases of movement to the better known forces and to 

 ignore the others. 



This bird's eye view of the forms of energy in the plant demonstrates to us 

 how far we are from having attained a comprehensive insight into the problem 

 of the transformation of energy. This is nothing more than what might have 

 been expected, since in the inorganic world also we have yet much to learn on 

 the subject. We are not, however, in a position to deny the validity of the law 

 of the conservation of energy in the organic world. 



In the following pages we will attempt to give a detailed exposition of the 

 various movements manifested by the plant. 



Bibliography to Lecture XXXI. 



BEIJERINCK. 1890. Mededel. Akad. Amsterdam. Natuurk. II, 7. 

 BIEDERMANN, W. 1895. Elektrophysiologie. Jena. 

 BONNIER. 1893. Annal. Sc. nat. VII, 18, i. 

 BURDON-SANDERSON. 1888. Phil. Trans. 179, 417. 

 COHN. 1893. Ber. d. bot. Gesell. n (66). 

 DELPINO. 1870. Comp. HILDEBRAND, Bot. Ztg. 28, 590. 

 DUTROCHET. 1840. Annales Sc. nat. II, 13, 5. 

 ERIKSON. 1881. Unters. bot. Inst. Tubingen, i, 105. 

 GARREAU. 1851. Annales Sc. nat. Ill, 16, 250. 



