IN VEGETABLE BIOLOGY. 233 
prothallus has lain in the shade two hours longer. Here the 
progress towards epistrophe is still more marked; but the edge- 
wise grain has not yet obtained sufficient room to permit of its 
turning face-up. Similar in every respect was the behaviour of 
the grains of Funaria hygrometrica leaves moving into positive 
apostrophe, and abutting upon their fellows which had already 
reached the wall (Pl. VII. fig. 12); as also those of Vaucheria sp., 
and in the long barrel-shaped cells of the leaves of Echeveria 
metallica edge-set grains were seen many hours after withdrawal 
from direct sunlight (fig. 18). 
It must be remarked that in the cells of plants with slow- 
moving protoplasm it is much rarer to find edgewise grains than 
. in those of Elodea; this is only what would be expected when it 
is remembered that variations in pressure at different points of 
the plasma are necessarily less frequent and less violent in the 
former types. 
But the coup de grace is given to Stahl's doctrine by the fact 
that the grains of darkling plants may occasionally be found 
upon their edge. I have seen this in examples of Elodea and 
Vallisneria set in the dark for several days; and at fig. 10 is 
shown an edgewise grain in a clump massed in an epidermal cell 
of Pteris serrulata kept in darkness for three weeks; this last 
Period is, as has already been seen, sufficient to cause the apos- 
trophized grains of Funaria hygrometrica to come out upon the 
surface-walls (negative epistrophe), in which position they can 
here and there be found tilted up on their edge, it being some- 
times possible to refer the peculiarity directly to the effects of 
Pressure exerted by surrounding grains (fig. 4). The negatively 
apostrophized grains of higher plants may also sometimes be 
found in profile in places where they abut upon their fellows. 
The Law of Positive Progresswn. 
It has before now been noticed* that the grains of a plant 
brought out of darkness into diffused light more quickly pass 
into epistrophe than they do into apostrophe when removed from 
diffused light into darkness. This rule I find to be, so far as 
My experience goes, of universal application ; and not only so, 
but the fact is but a part of what seems to be a general law. 
Not merely is it true that more time is required to induce nega- 
* First by Borodin, in 1867. 
