WATER, ITS PROPERTIES AND FUNCTIONS 241 



selves with a layer of substance comparatively impermeable to water, forming 

 what are known as "cysts," as mentioned above in reference to Rotifers. 



It will also readily be understood that, in the dry state, protoplasm can 

 withstand freezing temperatures better than in the normal active moist state. 

 Seeds, although they are not absolutely devoid of water, can be exposed to 

 the temperature of liquid air without injury. 



HYDROTROPISM 



The need of water causes certain organisms to turn towards the place where 

 it is to be found. This fact is very marked in the case of roots, leading to the 

 phenomenon known by the above name. The side of roots turned away from 

 the water grows more rapidly than that turned towards it, so that curvature 

 results. The opposite behaviour is shown by the sporangia of Mucor, leading 

 to bending away from the moist surface. 



THE VISCOSITY OF LIQUIDS 



The subject of viscosity is, strictly speaking, not quite in place here, since 

 it concerns other liquids in addition to water. But since, in physiological 

 work, the liquids with which we have to deal are, almost entirely, solutions 

 or suspensions in water, we may be allowed to take the subject at this stage, 

 as a convenient one. 



As was pointed out by ^Newton, the particles of liquids are not free to move 

 about without resistance due to their "adherence" to one another. This gives 

 rise to friction, so that the viscosity, or internal friction, of a liquid is proportional 

 to the velocity with which these particles are moving past one another and also 

 to the extent of the rubbing surfaces. 



The methods used for its determination consist either in measuring the resistance 

 offered to the movement of a surface passing through the liquid, or in that of 

 the resistance offered to the passage of the liquid through a narrow tube ; the 

 latter method is a simple one and requires merely the determination of the 

 time taken by a given amount of the liquid, under a given pressure, to run 

 through the tube. 



The flow through tubes is not only the most important aspect of this property 

 of liquids met with in ordinary life, but also in physiology, where the internal 

 friction of the blood gives rise to what is often called the " peripheral resistance " 

 of the vascular system. This it is, that, with a given rate and strength of heart 

 beat, determines the arterial pressure. 



The first point to be noted is, that when a liquid is being caused to flow through 

 a tube by the pressure applied at the inlet end of the tube being greater than that 

 at the outlet, the layer in immediate contact with the wall of the tube is at rest, 

 while that in the middle has the greatest velocity ; each layer experiences friction 

 at its contact with the neighbouring layer, so losing in velocity progressively until 

 the outermost layer is reached, where the velocity disappears entirely. We see, 

 then, that the friction is between the parts of the liquid itself and riot between 

 the liquid and the wall of the tube. 



Suppose, next, that the tube is a wide one and that the internal friction of 

 the liquid is not great ; the thickness of the layer at the periphery in which the 

 velocity is increasing from zero to its maximum rate will only be a narrow one 

 The remainder of the column moves in all its parts with the same velocity, so 

 that, in this part of the stream, there is no friction. Such tubes are the large 

 arteries and veins. In a narrow tube, such as an arteriole, the layer whose 

 constituent elements are in motion relatively to one another will reach to the 

 axis of the tube, so that the whole of the liquid column is exposed to internal 

 friction. We see, then, how, even supposing that the number of arterioles into 

 which a large artery divides is sufficiently great to give a total cross-sectional 

 area equal to that of the large artery, so that the rate of flow is no greater, 

 the total mass of blood is causing fractional resistance, whereas in the large 



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