9] 
and hence it is safe to conclude that the optimal intensity of 
illumination for the chloroplastid itself is considerably weaker 
than that of direct sunlight. This is certainly the case when 
the exposure is prolonged, for my own researches have shewn 
that a chlorophyllous cell enclosed only by a transparent 
cellulose wall can not be exposed to full sunlight for a day 
with impunity, while in many cases such exposure is sufficient 
to kill and bleach the whole of the chloroplastids. Since the 
above experiments by Giiray lasted for periods of 2 hours only, it 
seems probable that here we are dealing with the direct effects 
of the strong illumination, but in all cases it would be ne- 
cessary to determine whether the lessened inability to assimilate 
is a direct effect of the more intense illumination, or whether 
it is due to a more or less complete condition of assimilatory 
inhibition having been induced in the chloroplastids '). 
The protective paraheliotropic movements which the leaflets 
of many Leguminosae, and the leaves of other plants as well, 
undergo when exposed to direct sunlight are of great interest 
and exhibit many features worthy of further study. Thus in 
many cases the degree of irritability of the sensitive pulvinus 
and the rapidity of the response produced have yet to be 
etermined, while the extremely large and slowly reacting 
pulvini of Bawhinia afford exceptionally favourable experimental 
Material for the study of various points in connection with 
= pulvinar irritability. Thus it is not yet certain whether _ 
stimulus acts directly upon the cortical pulvinar tissues which 
Y altering their turgidity cause the curvature, oF whether 
the epidermis or some special layer of cells acts as the per- 
“plent Organ and transmits the stimulus to the reacting 
tissues, Certain facts already given (1. ¢ p. 447) seem . 
_ Indicate that the former is the case, but the question could be 
re Teadily determined directly in Bauhinia by removing the gee 
*xposing the pulvinus to sunlight either in a moist atmo- 
0 age 
) Leaves 
Totes Similate less 
n strong sunlight may 
p. 68, Cassia timorensts. 
+ = See er ee 
which assume a paraheliotropic position i 
actively on this account alone. See Giity, |. ¢., 
y OSE Bee Pte Sine fan am lta Sorel (5 
; 
