468 Dr. A. J. Ewart. On the Physics and Physiology 



required to produce moderately rapid streaming through the pores. 

 The differences of hydrostatic pressure between the different seg- 

 ments of an intact sieve-tube would, however, frequently suffice to 

 produce a direct transference in mass of the fluid contents through the 

 pores of sieve-tubes at a millimetre or so per minute,* for, as evidenced 

 by the influence of gravity and of centrifugal force upon suspended 

 particles, the viscosity of the contents of the sieve-tubes is relatively 

 low. 



In the case of bacterium cells of 2 to - 5 /A diameter, it is very 

 doubtful whether the protoplasm could with economy generate the 

 required propulsive force for perceptible internal streaming movements. 

 The latter take place in diatoms mainly on the external surface, and 

 require relatively little expenditure of energy. 



In the case of protoplasmic threads passing through fixed channels in 

 the cell-wall, if these are y^ //. diameter and 5 /A length, a pressure of over 

 6 atmospheres would be required to drive a liquid of viscosity O075 

 through a single thread at a velocity of 1 mm. per second. So that if 

 a tissue-cell were isolated, it would at the lowest computation take 

 several years for the escape of 1 cubic mm. of the cell-contents through 

 one such thread under a maximal internal osmotic pressure, even sup- 

 posing that the thread did not become plugged by floating particles and 

 that no cellulose was formed across its exposed end. The outer layers 

 of the ectoplasm appear, moreover, to be very much more viscous than 

 the endoplasm, and hence it is obvious that the threads do not subserve 

 purposes of translocation, although minute particles of protoplasm may 

 be bodily transferred from one cell to another in the course of time. 



The direction of streaming is mainly determined by internal factors, 

 and in rotating cells a reversal is only possible in certain cases and 

 under very special conditions. 



Changes occur spontaneously however in cells exhibiting circulation. 

 The total resistance during circulation is greater than during rotation, 

 and hence, unless the velocity increases considerably, a change from the 

 former to the latter after stimulation is not due to an increased energy 

 of streaming but to a change in the configuration of the protoplasm. 



The energy for streaming can be derived either from aerobic or 

 anaerobic metabolism. Certain species of Cham and Nitella are in fact 

 facultative anaerobes, and may exhibit slow streaming for 6 8 weeks 

 in the entire absence of free oxygen. 



No special chemical changes are connected with streaming. 



* .According to de Bary (' Compt. Anafc.,' 1884, p. 177) finger-like processes 

 from the adjoining segments of a sieve-tube meet in the sieve-pores, but remain 

 distinct. This discontinuity lias only been observed in dead sieve-tubes, and it 

 probably results from the breaking of the viscous protoplasmic threads at their 

 thinnest points on death, which is a surface-tension effect commonly produced in 

 dying protoplasmic threads. 



