SYNTHESIS, STORAGE AND BREAKDOWN 133 



confirmation in more critical experiments, which have revealed a 

 close correspondence between the daily fluctuations in the concen- 

 tration of carbohydrates and nitrogenous substances in the leaf-cells, 

 and those in the sieve-tubes of the stem below the leafy zone. 



The movement of soluble materials through the sieve-tubes is 

 not a matter of simple diffusion, although it has certain resemblances 

 to a process of that type : such as the fact that often, though not 

 always, translocation occurs from regions of higher concentration 

 of a particular substance to those of lower. But the rate of trans- 

 location is greatly in excess of that which could be accounted for by 

 diffusion alone. One suggestion is that passage of material through 

 the sieve-tubes is hastened by a circulation of the protoplasm. But 

 this phenomenon, though fairly common in plants, is not regularly 

 observed in sieve-tubes. Another suggestion calls in the aid of 

 osmotic forces. At present, however, we have no definite information 

 of the forces productive of translocation. 



Though the sieve-tubes provide the main channels for translocation, 

 it is clear that since relatively few cells are in direct contact with those 

 elements, translocation from one organ to another must involve 

 in both organs a cell-to-cell movement of materials in tissues other 

 than the phloem. It has been suggested that materials in solution 

 may pass rapidly from cell to cell through the finer protoplasmic 

 connections (see p. 26), as they are known to do through the larger 

 protoplasmic connections traversing the sieve-plates. 



Respiration. 



A living organism requires a constant supply of energy. The 

 most obvious way in which energy may be used in a plant is in the 

 various chemical syntheses ; in the particular instance of photo- 

 synthesis the energy of light from the sun's rays can be utilised 

 directly, but for the rest, and for other vital activities, an internal 

 source of energy is necessary. The readiest source of energy 

 available for the ordinary purposes of men is by the combustion 

 of fuel, such as coal or wood : that is, its oxidation, the ultimate 

 products being carbon dioxide and water. The latent energy of the 

 fuel is converted on combustion into kinetic energy. This supplies 

 the motor impulse for engines of various kinds. In a somewhat 

 similar way there proceeds, within the cells of the living plant or 

 animal, what may be described as a slow physiological combustion 



