126 



PRINCIPLES OF GENERAL PHYSIOLOGY 



A further practical point of some importance is that, when a substance is 

 found to penetrate into a cell, the conclusion must not hastily be drawn that 

 the cell is normally permeable to this substance. The experiments of Osterhout 

 (1912), in which the cells of Laminaria were found freely permeable to sodium 

 chloride when this salt was present alone, but impermeable to it when calcium 

 was also present, are sufficient to prove the contrary. In fact, statements 

 regarding permeability to any particular substance can only be held to be valid 

 when the proof is given that the membrane is in its normal state, a proof that 

 is not always given, and one which, as must be confessed, it is not always 

 easy to give. 



There are certain other substances, in addition to electrolytes, which produce 

 changes in permeability. The most important of these are those known as 

 anaesthetics or narcotics and will be discussed in a succeeding section of this 

 chapter. 



Certain functional states of the cell are known to be accompanied by changes 

 of permeability ; the state of excitation produced by stimuli in contractile tissues 

 appears to be accompanied by increased permeability to electrolytes ; this will 

 be discussed later. 



Lepeschkin (1908) finds that the permeability of plant cells is increased 

 by exposure to light. The question was worked out further by Trondle (1910), 

 especially with respect to the relation between the amount of change and the 

 intensity of the illumination. The bearing of this fact on the explanation of 

 the movements which take place under the action of light is obvious. Diminu- 

 tion in permeability produces a fall in the concentration of osmotically-active 

 substances in the cell, the osmotic pressure and turgor consequently fall in 

 value, so that opposing forces are able to bend the side of a stem exposed to 

 light. Hence the heliotropic curvature. V. H. Blackman (1914) also finds 

 that light causes increase of permeability in the pulvinus of the sensitive plant, 

 described on page 431 below. 



Again, the great variation in the relative concentration of sugar in the blood 

 corpuscles and the plasma, and the manner in which changes in the concentra- 

 tion in the plasma affect that in the corpuscles, serve to show that the permeability 

 of blood corpuscles is not a fixed and unalterable thing. The following data 

 from a paper by Sober (1912, i.) will illustrate the point: 



An increase of glucose concentration in the blood was produced in various 

 ways, adding glucose to shed blood, and determining the distribution between 

 plasma and corpuscles after standing, giving adrenaline to the living animal, 

 extirpation of the pancreas, or a large amount of glucose introduced into the 

 stomach. 



