FORMS OF ENERGY IN THE PLANT 403 



From the facts cited we must conclude that differences in electric potential 

 appear in all organs of the plant, wherever adjoining parts exhibit chemical or 

 physical differences, and such differences may exist between parts of the same 

 cell or between individual cells or complete tissues. 



Detailed information as to the causes of these electric phenomena is not 

 forthcoming, and we will content ourselves with these scanty remarks, since 

 not even guesses as to the significance of such electric currents can be made 

 (compare BIEDERMANN, 1895). 



Amongst the varied activities of the plant the production of mechanical 

 energy is, as we have already remarked, by far the most prominent, and the 

 movements which are the expressions of the expenditure of mechanical energy 

 have been much more accurately studied than electric, thermic, or photic 

 phenomena. The rest of the present course of lectures will be devoted to a dis- 

 cussion of these movements. We are already familiar with some of these 

 movements, for when we dealt with the absorption and distribution of materials 

 we incidentally studied the movements of these substances in the plant, a subject 

 we dealt with in several lectures in the first section of this work. We have now 

 to study other movements, e. g. the free locomotion of the lower plants, proto- 

 plasmic streaming which takes place in a cell and which is analogous to these 

 movements, and, finally, the innumerable varieties of movement seen in fixed 

 organs. In all these movements the plant has to overcome resistance, internal 

 and external and to do work. Without inquiring more intimately at present into 

 the nature of the various movements we may here appropriately summarize the 

 information we possess as to the source of the energy used by the plant in carry- 

 ing out these movements. 



We must first of all make inquiry with regard to the chemical energy which 

 undoubtedly plays an important part in these movements. It is true that the 

 role is partly an indirect one, in so far as without the chemical energy set free in 

 respiration it would be impossible to construct the plant or renovate the 

 apparatus which carries on the movement. But we cannot doubt that the 

 energy released during the decomposition of food material co-operates directly, 

 since the endless manifestations of movement are most intimately connected 

 with respiration and stop short at once when intra-molecular respiration begins 

 in ordinary plants. Having established that fact we need only add that respira- 

 tion is an indispensable condition of protoplasmic movement and that it fur- 

 nishes the energy necessary for it (PFEFFER, 1892). Again, we have learned else- 

 where to recognize the existence of necessary factors which act as stimuli only. It 

 is a fact, however, that the majority of stimuli also bring to the organism a cer- 

 tain amount of energy, but it is characteristic of these stimuli that their energy 

 stands in no relation to that of the effects they produce. The energy of the 

 stimulus may be much less or much greater than that of the movement released, 

 and the latter is certainly not produced from the stimulus but from the stores in 

 the plant itself. Respiration may also be only a releasing stimulus and it cer- 

 tainly is so in many cases, though it is probable that it frequently has a direct 

 energizing significance, or, in other words, that the chemical energy released may 

 be transformed directly into mechanical energy. It is impossible, however, to 

 demonstrate this view. In treating of respiration we are accustomed to compare 

 the energy evolved with that given off in other cases of combustion. In such 

 cases as, e. g. the burning of wood or coal in a steam-engine, we encounter at 

 first a transformation of chemical energy into heat, and it is the heat in the 

 first instance that does the work. In the plant, however, as we have seen, heat 

 as such, evolved in respiration, plays no great part, since it cannot be replaced 

 by heat produced by other means. But even though exact proof were forth- 

 coming that respiration had a purely energizing significance such proof would 

 still fail to satisfy us in the absence of information as to how the chemical 

 energy is transformed directly into mechanical. 



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