James Small 
76 
Ewart 1 states that “we have weighty reasons for considering the 
viscosity of the main bulk of the streaming protoplasm to be within 
the limits *04 to -2 at 18 0 C.” Since Siefriz 2 has shown that the 
active protoplasm of young cells has a minimum viscosity (i.e. some¬ 
thing below *037), it is quite reasonable to take Ewart’s lower limit 
(•04) as the viscosity of the protoplasm in the meristem. (4) Pro¬ 
fessor Blackman next states that my conclusion that the “ creaming ” 
is governed by Stokes’ Law is certainly mistaken. As pointed out 
above (p. 72) the application of Stokes’ Law is commonly regarded 
as an accurate method of measuring the rate of fall of particles 
(even down to -02 /x diameter) which are heavier than the medium, 
and there is no reason to suppose that a density difference in the 
opposite sense renders futile the application of a general law like 
that of Stokes. Although the ultimate distribution of even very 
heavy particles is determined by the exponential “rarefaction law,” 
the initial stages of fall must be governed by Stokes’ Law. Perrin 
makes this point clear, when he states (op. cit. p. 35) that: “It is 
necessary to employ a capillary tube to avoid the convective move¬ 
ments ” which occur in wider tubes; that in “a shallow cylindrical 
vessel about 100 /x in height” (op. cit. p. 31), “if our kinetic theory 
is exact, this [uniform] distribution will change from the time the 
preparation is left at rest” (op. cit. p. 41); and that this change is 
obvious in a few minutes (see above). Perrin is even more explicit 
in the extrapolation from Stokes’ Law which he gives, adding (op. 
cit. p. 40) that: “The preceding experiments show that this law is 
valid in the domain of microscopic quantities, and the verifica¬ 
tion pushed even to the threshold of ultramicroscopic magnitudes, 
scarcely leaves a doubt that the law may still be valid for the far 
smaller granules of ordinary colloids, or for the large ions found in 
gases.” And further (op. cit. p. 76), “Now the reasoning of Einstein 
supposes the law of Stokes to be valid. It is therefore probable that 
this law, the exactitude of which I have proved directly as far as 
dimensions of the order of a tenth of a micron (No. 21), still remains 
exactly verified for large molecules, the diameter of which does not 
reach the thousandth of a micron. It will permit us presently to 
apply the law of Stokes with safety to the case of ions in movement 
through a gas.” 
Stokes found that his law is valid so long as the radius of the 
1 On the Physics and Physiology of Protoplasmic Streaming in Plants, by 
A. J. Ewart, p. 19. Oxford. 1903. 
2 Botanical Gazette, 70 , p. 360, Nov. 1920. 
