78 James Small 
legitimately take as being ♦ 04 (see above, p. 76). Then applying 
Stokes’ Law, we find the rate of creaming to be 1*55 /x in the 7 
minutes of Presentation Time 1 with particles of -2 ft radius, and 24-8/x 
in the same time with particles of -8 /x radius. Since the particles 
begin to cream almost immediately, when the organ is laid hori¬ 
zontally, a very short Perception Time can be detected by suitably 
delicate apparatus 2 . 
An interesting point is that, with short periods of intermittent 
stimulation on a klinostat, each particle would remain for some 
time during rotation very nearly 3 at the point which it had reached 
during each successive stimulation; and no curvature would take 
place until the total potential difference produced by the “ creaming ” 
particles was sufficiently strong to develop in its turn a turgor 
difference (between upper and lower sides of the organ) which would 
be sufficient to overcome the mechanical rigidity of the organ. In 
this way we arrive at some sort of explanation of the occurrence 
of Presentation Time and Relaxation Time 4 . Therefore, whereas it 
may “seem impossible to correlate active electrical and mechanical 
reaction occurring in a few seconds or minutes with a redistribution 
of plasma particles requiring days for its accomplishment,” when 
we assume that at least some of the particles are near but not under 
the limits of microscopic visibility, we can carry the correlation 
of the redistribution of such particles with geotropism into 
practically all the details of that most obscure physiological 
phenomenon. 
In the third section of his article Professor Blackman raises 
another difficulty, namely the disappearance of potential differences 
when the “creaming” is completed. It is quite reasonable to sup¬ 
pose that protoplasm, like most colloidal solutions and emulsions, 
contains dispersed particles which vary considerably in size. The 
larger microns are supposed to bring about the rapid perception 
and the initial action current by their relatively quick “creaming.” 
But Professor Blackman so ably expounds in the earlier sections 
of his paper the slow rate of creaming of the smaller particles, that 
it is scarcely necessary to point out that these submicrons, too, may 
be taken as electrically charged; and that by their slow creaming, 
1 For the broad-bean root. 2 See Proc. Roy. Soc. B. 90 , p. 351. 1918. 
3 The “outward” movement according to Gibbs’ Law being slower than 
the “creaming.” 
4 Would heavy particles which stick and then produce “ a sudden precipita¬ 
tion of geo-electric response” show any definite Relaxation Time? 
