The Royal Seciety, London. 
117 
plant is reversecl in position in regard to light, the stomata rapidly 
accommodate themselves to the change in illnmination. 
The most interesting fact in regard to the effect of artificial 
darkness is that it is more effectual in prodncing closure in the 
afternoon than in the morning; and, couversely, illnmination opens 
closed stomata more readily in the morning than later in the day. 
These, together with other observations, tend to show a certain 
amount of inherent periodicity in the nocturnal closure of the 
stomata. Another fect of interest is that in darkness prolonged 
for several days the stomata gradually open. This last Obser¬ 
vation is nsed in the section on the mechanism of the stoma as 
an argument against the prevalent view that the stoma closes in 
darkness, because in the abeyance of assimilation the osmotic 
material, on which the turgor of the gnard cells depends, ceases 
to be manufactured. 
Schellenberger 1 ) has striven to uphold this view by showing 
that in the absence of CO 2 the stomata close as though they were 
in darkness. My experiments on plants deprived of CO 2 lead to 
absolutely contrary results, namely, that the stomata remain per- 
fectly open even during prolonged deprivation of CO 2 . 
It is a vexed question 2 ) whether or no the majority of plants 
close their stomata at night. My conclusion is that in terrestrial 
plants (excluding nyctitropic plants) a great majority show some 
closure at night; the horn hygroseope Stands at zero on the stomatal 
surface of by far the greater number of ordinary plants. On the 
other hand, the hygroseope shows widelv open stomata on most 
aquatic plants at night. Stahl 3 ) concludes that nyctitropic plants 
are remarkable for not closing the stomata at night; this fact I 
somewhat doubtfully confirm; but the question is not so simple as it * 
seems owing to the varying behaviour of the stomata at night in 
different temperatures. 
Since the hygroseope gives numerical readings it is possible 
to repressent graphically the daily opening and closing of the 
stomata. The curve begins to leave the zero with the morning 
light; it rises rapidly at first, and afterwards more slowly. In 
some caces it runs roughly horizontally until a rapid fall begins 
in the evening. In other cases there is a slow rise up to the 
highest point, which occurs between 11 A. M. and 3 P. M. The 
hygroseope generally sinks to zero within an hour after sunset. 
The effect of heat has not been fully studied, but enough 
has been done to confirm previous observers who find that heat 
opens the stomata. As regards the visible spectrum, I find that 
the red rays are decidedly most efficient, but I am not able 
to find any evidence of a secondary maximum in the blue, such 
as K o h l 4 ) describes. 
B Bot. Zeitung. 189G. 
2 ) Leitgeb, Mitteilungen aus dem Bot. Inst, zu Graz. 1886. 
3 ) Bot. Zeitschrift. 1897. 
4 ) Beiblatt zur Leopoldina. 1895. 
