236 MR. 8. LE M. MOORE’S STUDIES 
placed, whether the upper layer receive the first impact of light 
or vice vers&, provided that the leaf be healthy, rotation is more 
rapidly set up in the cells of that layer. The statement is also 
true, as is well known, of pieces of Vallisneria-leaf, whether slit 
down longitudinally or not, only here the area of readier disturb- 
ance is that occupied by the large cells, many times longer than 
broad, which are found beneath the surface. In the case of 
Elodea photolytic difference between the two layers was held to 
be largely due to difference in the tone of the protoplasm with 
regard to light, because other possible causes of discrepancy, 
such as variation in the relation between protoplasmic momentum 
and number and size of chlorophyll grains, as well as in the form 
of the cells, were here absent; and I am disposed, for want of a 
better, to extend this idea so as to embrace the facts of rotation 
in this type. But it seems certain that the notable difference in 
the form of the small outer and the much longer inner cells of . 
Vallisneria must exert some influence upon rotation. For the 
stream of protoplasm, slowly moving along the side of a long 
cell, will not only undergo the diminution of its velocity con- 
sequent on turning a corner less frequently than must necessarily 
happen in a small cell, but the uninterrupted flow of the stream 
down a long side will enable it to acquire suffieient momentum 
to carry it round a corner sooner than would be possible in a 
small cell *. Besides this, it must be remembered that there is 
relatively less chlorophyll in the inner than in the outer cells, 
and, consequently, less inertia to be overcome by the protoplasm. 
These considerations are, perhaps, applicable without reserve to 
the long narrow midrib-cells of Hlodea-leaves, in which rotation 
sets in a little sooner than it does in the surrounding tissuef; 
but the very great diversity in this respect between the large 
and small cells of Vallisneria would seem to denote a difference 
in the tone of the protoplasm as being, perhaps to a less extent 
than in Elodea, contributory to the phenomenon in question. 
It is well known thatrotation in detached leaves of Elodea can 
be stopped by their removal to darkness; but wliat has not been 
explained is the varying length of the time required to bring 
* Velten has shown that the protoplasmic stream tends to flow in the direc- 
tion of least resistance (see ‘ Flora,’ 1873, p. 87). 
t It is worthy of mention that positive apostrophe is more readily induced 
E the long narrow cells of the midrib of leaves than in the cells of the 
mina. 
