216 MR. S. LE M. MOORE’S STUDIES 
critical point of L. trisulca has been fixed with, it is trusted, fair 
accuracy by studying the behaviour of specimens kept for several 
days in light of different intensities, each intensity estimated, as 
cautiously as possible, in terms of the photrum. But far different 
is the case with ordinary aquatic and shade-loving plants, the 
grains of which require several days or even weeks to pass into 
negative apostrophe; the only way to determine the negative 
critical point of these is to keep them for some time in very low 
light. Treated in this way, it was found that healthy Elodea- 
plants, exposed to the lowest illumination for a fortnight, showed 
no signs of apostrophe; it is therefore safe to affirm that the 
epistrophic interval of this type abuts on darkness. It must not 
be forgotten that large approaches to apostrophe are observed 
under the above conditions if the specimens be old and weak or 
fragmented. Want of time and paucity of material have pre- 
vented my fully dealing with the other types of this set. 
u Fig. 2. 
bad 
a g 
1. Funaria hygrometrica (leaves). 
2. Garden Chrysanthemum (involucral scales). 
3. Urtica urens (stipules). 
4. Poa annua (leaf-sheaths). 
The next diagram (fig. 2) shows the epistrophic interval of four 
land-plants, viz. Funaria hygrometrica (leaves), Pyrethrum sinense 
(involucral scales), Urtica urens (stipules), and Poa annua (leaf- 
sheaths). The positive critical point, which in the first three of 
these extends to the right-hand extremity of the photrum, was 
determined upon two hours’ exposure under conditions similar 
to those before mentioned ; the negative critical point was found 
from specimens set in darkness overnight, and laid out in various 
positions in the photrum for between three and four hours (the 
time required by the grains to move into epistrophe) immediately 
after removal from the dark. The negative critical point of the 
Poa-grains occupied the same position as that of the others, but 
