MR. A. J. EWART ON ASSIMILATORY INHIBITION. 443 
shown that young leaves of Amherstia nobilis exposed horizon- 
tally to full sunlight formed on extraction with alcohol a weaker 
solution of chlorophyll than shaded leaves or leaves kept in the 
normal drooping position did. 
Pringsheim * has shown that concentrated sunlight exercises 
an oxydative photo-chemical effect upon plant-cells and chloro- 
phyll, bleaching the latter, and inducing in the cell-plasma a 
condition of light rigor, finally followed by death. 
Thus exposure to concentrated sunlight, precautions being 
taken to avoid all heating effect, and hence to obtain a purely 
photo-ehemical action, causes the rotation of protoplasm to be 
slowed, and finally stopped in a condition of light rigor, from 
which at first recovery is possible, but not later (after 5 to 10 
minutes” exposure). It is possible to produce a condition of 
permanent light rigor, z. e. death, in the chlorophyll grains over 
an exposed region of a cell of Chara without affecting the 
vitality of the cell, i. e. the plasma of the chlorophyll grain 
appears to be more sensitive than the general protoplasm of the 
cell. Hence the temporary cessation in the power of assimi- 
lation of chlorophyllaceous cells, which may be caused by pro- 
longed and continuous exposure to direct sunlight, is probably 
due to a photo-chemical effect upon the plasma of the chloro- 
phyll grain, producing in it the initial stages of light rigor, 
associated with a temporary inability to assimilate, the action 
being similar to the paratonic influence of light upon growth, 
and the condition induced in the ehlorophyllous plasma being 
perhaps analogous with the condition of immotility which may 
be produced in the protoplasm of irritable organs by the action 
of various chemical and physical agencies. 
However the hindrance or stoppage of assimilation may be 
produced, the fact that such may occur as the result of prolonged 
exposure to direct sunlight gives additional interest to the dif- 
ferent means of protection which various plants possess against 
the action of light. In temperate regions it is only on very rare 
occasions and under exceptional conditions of exposure that 
any marked influence upon assimilation is produced in healthy 
adult leaves, and hence here protective agencies are almost 
entirely absent. The leaflets of the shade-loving Ovalis Aceto- 
sella, however, when exposed to sunlight assume a drooping 
* Pringsheim, “Zur Theorie der Assimilation, Chlorophyllfunction und 
Lichtwirkung,” in Pringsh. Jahrb. Bd. xii. 1882, pp. 326-344. 
LINN. JOURN.—BOTANY, VOL. XXXI. 21 
