IN VEGETABLE BIOLOGY. 213 
from each of the following types, growing in low diffused light, 
viz. Funaria hygrometrica, Callitriche verna, Elodea canadensis, 
and Lemna trisulca, and mounted in water on glass slides, after 
remaining under uniform conditions of illumination close to the 
window of a room for two hours or thereabout, it may happen, 
provided that the light be of the exact quality required, that the 
chlorophyll of Callitriche will be in epistrophe, while in the other 
three it will be in apostrophe*. Should now the weather clear 
up, so that the leaves are for some time exposed to good diffused 
light, the grains of Lemna will remain in apostrophe or show 
more or less tendency towards massing in the corners of their 
cells; those of Elodea will rotate round the cell-periphery ; in 
Callitriche they will be found in apostrophe; while epistrophe 
will be established in Funaria. To superficial observation the 
simultaneous apostrophe of the chlorophyll of Funaria and of 
Elodea would seem due to the same cause; the remarkable fact 
of its dependence in the former plant on deficiency, in the latter 
on excess of light, would, one might safely say, never be even 
suspected. 
It is therefore plain that if a graphic representation, as exten- 
sion in space, of the whole range of possible grades of illumina- 
tion from darkness to direct sunlight be drawn upon a uniformly 
reduced scale, it will be found that the portion of the scale re- 
presenting the various intensities of illumination sufficient to 
bring out negatively apostrophized grains into epistrophe and 
insuffieient to drive epistrophized grains into apostrophe will 
depend very much, in respect of extent and position, upon the 
plant which is made the subject of experiment. To this portion 
of the scale, comprising all epistrophizing grades of illumination, 
it is proposed to apply the term “ epistrophie interval”; for the 
whole scale the word “ photrum " may, perhaps, be allowed as at 
once simple and convenient. It goes without saying, that great 
difficulties stand in the way of the proper construction of the 
photrum. In the after-mentioned experiments the photrum was 
twelve feet long; the end nearest to sunlight (which it is pro- 
posed to call the “ positive” end) was close to the only window 
Of a room from which the sun's direct beams were excluded by 
* In some of the Elodea-cells a few of the grains may be in epistrophe, either 
solitary or grouped. This is exceptional if the light be all that is required for 
the success of the experiment. 
LINN. JOURN.— BOTANY, VOL, XXIY. T 
