THE ENERGY EXPENDED IN A STREAMING CELL 



29 



But an actively respiring seedling may produce from i to 2 per cent, 

 of its weight of CO 2 per day, which corresponds roughly to a consumption 

 of 0-017 to 0-034 gram of cane sugar per day per gram of protoplasm. 

 Hence in cells approaching o-oi cm. diameter, even if only I per cent, 

 of the energy directed towards streaming is actually utilized, the whole 

 amount expended does not form more than T ^-^ of the energy represented 

 by moderately active respiration. In ordinary plant-cells, therefore, it 

 seems certain that much less than a tenth of a per cent, of the energy of 

 respiration is consumed in producing streaming movements. 



In the case of a tube of o-ooi cm. internal diameter, a consumption 

 of TOO times as much energy would be required as in one of o-oi cm. diam. 

 to produce the same velocity of flow. This might represent as much 

 as from ^ to i per cent, of the energy of respiration, and herein probably 

 lies the reason for the absence of streaming movements in small young 

 cells, and their gradual commencement and increase in velocity as the cell 

 grows larger and older. In any case it is of importance to remember that 

 the influence of the diameter of the cell upon the resistance to streaming is 

 much greater than that of any possible changes of viscosity, the ratio 



between the two factors being as : r 7- 



Resistance to Flow in Protoplasmic Threads. 



In the case of the interprotoplasmic connexions passing through 

 minute channels in the cell-wall, the diameter is excessively small, and 

 hence the resistance to flow very great. Suppose a thread to be -$ /u, 

 diameter and 5 ju, length, then a pressure of 6 atmospheres would be 

 required to move a liquid of viscosity 0-075 through it with a velocity of 

 i mm. per second, taking one atmosphere as equal to i,oco grams per 

 sq. cm. The viscosity of the ectoplasm is certainly very much higher 

 than the values given, and the minutest solid particle would suffice to 

 block the thread. The smallest difference of surface tension at the ends 

 or middle of the thread would interpose a very great resistance to flow. 

 Thus if a tissue-cell were isolated in air it would need a pressure of 34 

 atmospheres to overcome the resistance due to the surface tension of 

 a drop of water escaping from a thread of y 1 ^ jw. diameter. Even in water 

 or in the intact tissue the total energy of respiration in the plasma of 

 the thread would hardly suffice to overcome the resistance due to these 

 various sources, although differences in the osmotic pressure of neighbour- 

 ing cells might be more efficient in this respect. But usually the osmotic 

 pressure in neighbouring cells differs only by a fraction of atmosphere, 

 and hence by this means it would hardly be possible to induce movements 

 in mass at a greater rate than i mm. per day. At this speed it would 



