IN VEGETABLE BIOLOGY. 367 
(i.) In diffused light the chlorophyll plate places itself at right 
angles to the incident light: very poor grades of illumination can 
excite this movement. ` 
(ii.) In sunlight the edge of the plate is turned towards the 
source of illumination. 
(iii.) On continued insolation, the plate, straight till now, 
assumes a curved figure. 
I am not quite sure if I rightly interpret Stahl's views on the 
first of these points. What I find is this:—If a Mesocarpus 
scalaris* plate be placed so as to receive from two sources rays 
of diffused light in planes at right angles to each other, and if 
it have originally been face up (fig. 195) to the rays having the 
greater intensity, it will remain in this position; but should its 
edge have been shown to the stronger rays (fig. 19 a), it will, 
after a short time, turn round upon its axis through 90°, so as to 
face them. Obviously there is more here than a mere setting 
broadside on to diffused light ; because if a similar experiment 
be arranged, but so that the lower intensity of the former occa- 
sion be the higher one of this, it will be found that the plate will 
turn so as to face these rays of higher intensity ; in other words, 
it places itself in such a position as to cut the greatest possible 
number of rays of diffused light of the highest illuminating value. 
There is something in this suggestive of the behaviour of a piece 
of soft iron in gathering up the greatest number of “ lines of 
force" when placed in a magnetic field. 
The time required by the movement to complete itself appears 
to vary inversely as the intensity of light. Thus, during the 
month of August last the following average values were ob- 
tained :— 
In good diffused light the plate swings round in 7 minutes. 
In medium » » » 15-20 ,, 
In lowest » » » 11-2 hours. 
To the dictum that the plate turns edge up to sunlight I have 
some objection to take. The result of many experiments made 
* I have called the subject of the following researches M. scalaris; and I 
believe the determination to be correct, In former years I have obtained 
M. pleurocarpus, De Bary (recognized immediately by the geniculate junctions of 
the threads), from the pond which furnished the specimens used this year, 
een which no junctions were found. But for this difference the two species 
are very much alike, except during conjugation, which I have not been fortunate 
enough to see in either. 
LINN. JOURN.—BOTANY, VOL. XXIV. 26 
