232 MR. 8. LE M. MOORE'8 STUDIES 
such spontaneous movement), but assumed that position in con- 
sequence of abutting on the clumps. But, so far as my own ex- 
perience goes, this is due to local disturbances in the mass of 
protoplasm which invests the grains composing tbe clumps, 
whereby abnormal pressure is caused at certain points. Even 
were not this a matter of easy observation, Stahl's doctrine is 
exceedingly questionable, for it might he asked with reason why, 
if the end of the movement is to guard the grain from undue 
action of the sun, should it be restricted to a small percentage 
of the grains? In fact, this edgewise position has nothing in the 
least degree peculiar about it, for a broadside grain in motion, 
when it reaches and strikes against the top of the wall at right 
angles to that upon which it has been moving, turns up on its 
edge just as would have been the case had it come in contact with 
aclump. Another instance of this is shown at fig. 14 (Pl. VIL.), 
which represents part of a cell with protoplasm in vigorous rota- 
tion, the plane of the rotating stream lying, as sometimes occurs, 
at one point in the plane of the leaf instead of perpendicular 
thereto. In this case, when the grain reaches the point where: 
the stream runs broadside to the observer, it is forced away from 
the side wall and immediately moves upon its own axis through 
an arc of. 909, reverting to the former position on again coming 
in contact with a lateral wall. 
Moreover, bearing upon this point is other evidence, less easy 
to collect, it is true, but not less conclusive. The more slowly- 
moved grains of ordinary plants may frequently be found when 
they collect into clumps set with their edge to the wall; and by 
keeping preparations under observation for some time, it is no 
very difficult matter to determine that here, as before, pressure, 
and pressure alone, is the cause of this. Figs. lla, b, c, show 
this; they are taken from the prothallus of a fern, probably a 
species of Adiantum. In the first of these we see the grains in 
the left-hand cell, under the direct influence of the sun, making 
for the wall, which several of them have reached; a second row 
abuts on the first, and has in consequence been forced to stand 
edgewise, while a few outer ones are still on their face. In the 
next figure, drawn after the prothallus has been for half an 
hour in the shade, the tendency of the grains is to move from 
the side wall, and all but one of them have turned on to their 
face, the exceptional grain remaining wedged in between its 
pr Fig. 11 c shows the position of the grains after the 
