CHEMICAL ENERGY 409 



in respiration each liberates exactly the same amount of energy as when 

 similarly decomposed or oxidized outside the plant *. 



SECTION 90. Special Cases. 



A few special instances may be discussed to illustrate the application 

 of the foregoing principles to concrete cases in which energy is consumed 

 in overcoming resistance, independently of whether the energy used is at 

 once dissipated or is in part stored up again for future use. 



ABSORPTION AND TRANSLOCATION 2 . Any unequal distribution, 

 however produced, tends to set up purely physical diffusion-currents which 

 ultimately restore equilibrium. It is immaterial whether the unequal distri- 

 bution is produced by the organism with or without a consumption of energy 

 by solvent enzyme action, or by the absorption or separation of soluble 

 constituents of the cell-sap. Since diffusion movements are extremely 

 slow, mechanical mixing and streaming movements become of great 

 importance in ensuring the rapid transference of substances from one 

 place to another 3 . Plants fixed to the soil are in part dependent upon 

 the movements of the surrounding air or water for a rapid supply of food- 

 materials. 



Diffusion and currents of wind carry carbon dioxide to the summit 

 of a tree, and the carbon accumulated there represents stored potential 

 energy without the tree having raised any portion of it to this height. The 

 same is the case when a dissolved substance diffuses upwards from the 

 roots, and even although the upward passage may be aided by mixing 

 or bending movements, by thermo-diffusion, or by convection currents, and 

 by upward streams of water produced by transpiration, none of these 

 necessarily involves any consumption of energy on the part of the plant. 

 In other cases, again, streaming and mixing movements resulting from 

 protoplasmic activity may aid in translocation without being essential, 

 although the translocation of dead or living materials through the pores 

 of sieve-tubes and through the inter-protoplasmic connexions of ordinary 

 cells could hardly take place without the aid of the protoplasm 4 . In 



' See Rubner, Die Gesetze des Energieverbrauchs bei der Ernahrung, 1902. Cf. also F. Mares, 

 Biol. Centralbl., 1902, Bd. XXII, p. 282. 



a Cf. Pfeffer, Studien zur Energetik, 1892, p. 268. 



3 [The rate of diffusion is more rapid than is usually supposed, especially when chemical 

 fixation aids in maintaining a high gradient of concentration along the path of the diffusion currents. 

 Even without this, less time is required for the complete diffusion of a dissolved salt through an 

 ordinary plant-cell than the protoplasm takes to stream around it when streaming is active. The 

 transference of a substance across a broad band of tissue by diffusion alone would, however, still bean 

 extremely slow process. Cf. Ewart, On the Ascent of Water in Trees, Phil. Trans., 1905, p. 40 of 

 reprint.] 



* [The inter-protoplasmic connexions of ordinary cells are of no importance in translocation. 

 Thus under normal conditions it would take 100 years for the transference of I cub. mm. of the cell- 



