416 Observations on Stomata. 



Schellenberger* lias striven to uphold this view by showing that 

 in the absence of C0 3 the stomata close as though they were in 

 darkness. My experiments on plants deprived of C0 2 lead to abso- 

 lutely contrary results, namely, that the stomata remain perfectly 

 open even during prolonged deprivation of CO 2 . 



It is a vexed questionf 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 hygroscope stands at zero on the stomatal 

 surface of by far the greater number of ordinary plants. On the 

 other hand, the hygroscope shows widely open stomata on most 

 aquatic plants at night. StahlJ 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 hygroscope gives numerical readings, it is possible to 

 represent 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 cases 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 hygroscope 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 Kohl describes. 



The biology of the nocturnal closure is a subject which can 

 hardly be discussed in a condensed manner. It is suggested that 

 the gaseous interchange of assimilation may require widely open 

 stomata, whereas respiration may be carried on with comparatively 

 closed apertures. If this is so, the stomata might be to a great extent 

 shut at night, and an economy in the use of water effected, without 

 detriment to metabolism. Observations are given to show that 

 quite another effect is brought about by nocturnal closure. As long 

 as the stomata are open, the transpiring leaf is considerably cooler 

 than the dry -bulb thermometer, but at night it has almost the tem- 

 perature of the air. In this way a saving of heat is undoubtedly 

 effected but it is not easy to say whether it is sufficient to be of much 



* * Bot. Zeitung,' 1896. 



f Leitgeb, ' Mittheilungen aus dem Bot. Inst. zu Graz,' 1886. 



j 'Bot. Zeitung,' 1897. 



' Beiblatt zur Leopoldina,' 1895. 



