398 TRANSFORMATION OF ENERGY 



such supplies of energy. It is quite otherwise with heat. It has akeady been 

 shown that all plant activity is conditioned by certain definite limits of tempera- 

 ture, but it does not follow that the heat of the external medium forms a 

 source of energy to the plant. There can be no doubt that the plant does 

 absorb heat from the exterior in cases where it exhibits a lower temperature 

 than the outer world, a condition of things by no means rare ; and if the plant 

 absorbs heat under such conditions obviously its energy must be increased. 

 We need not dwell on this fact, for it is self-evident ; what we must inquire 

 into is whether the plant requires such an addition of heat energy from without, 

 but to this question we can give no conclusive answer ; the probability is that 

 it does not require it. 



Probably, all the energy required by the plant is obtained in the form of 

 light and nutriment. The law of causality compels us to believe that all the 

 energy of the plant has entered it from without, for we cannot conceive of the 

 plant, any more than of non-living matter, creating energy within its own body, 

 so that what we have to investigate is the way in which the energy which 

 enters the plant becomes altered within it. Just as chemical compounds occur 

 in the organism which are manufactured in it only, so there may exist in it 

 forms of energy which occur nowhere else. Such specifically organic forms of 

 energy are, however, unknown. It must be confessed, however, that, we know 

 nothing of energies pecuhar to the organism. Even as to the changes which 

 occur in the energy entering the plant we know but little. Since only the 

 final stages in the process which manifest themselves externally are accessible 

 for investigation, we are confined to guesswork as to the transformations 

 taking place within. Among the final stages visible to us the most important 

 is mechanical energy. The movements which the organism, in whole or in 

 part, exhibits are obviously those most prominent and hence also most studied. 

 The production of heat must also be noted as a phenomenon of very wide oc- 

 currence, as to the meaning of which, however, we know very little, though we 

 are more conversant with its causes. In addition to heat, the production of 

 electric currents and of light in plants must be referred to, phenomena which, 

 as yet, can only be said to play a subordinate part in plant physiology. 



This third section of our lectures must deal almost exclusively with the 

 phenomena of movement, but before we commence their study we may briefly 

 refer to the other manifestations of energy met with in the plant, viz. heat, light, 

 and electricity. 



The temperature of the plant conforms, generally speaking, with that of the 

 exterior medium, sometimes it gives off heat to the environment, sometimes it 

 absorbs heat from it ; it is destitute of those special contrivances which are 

 found in warm-blooded animals for maintaining a constant temperature inde- 

 pendent of variations in the temperature of the surroundings. The temperature 

 of the plant may be lowered beneath that of the surroundings by radiation and 

 also by transpiration. If we desire to demonstrate the production of heat in the 

 plant it will be necessary for us to retard radiation and transpiration more 

 especially, also to prevent loss of heat by conduction. It will also be necessary 

 to prevent gain of heat by the plant, especially by insolation. Merely by 

 repressing transpiration one may often cause a plant organ, previously exhibiting 

 a temperature below that of the air, to attain a temperature considerably above 

 it. This may be accomplished most easily by selecting for experiment a massive 

 organ with relatively small superficial area, or by heaping together smaller parts 

 and surrounding them with a bad conductor. Thus the temperature of many 

 inflorescences exceeds that of the air very considerably, often as much as from 

 5° to 10° C. If germinating seeds, growing points, or flower-buds be collected 

 in a flask, and surrounded by a bad conductor, and if special care be taken that 

 oxygen gas can enter in sufficient quantity, considerable rises in temperature 



