June 30, i 



NATURE 



213 



must depend on a stomatal transpiration. The hygroscope 

 indicates well the gradual " closure"' of the stomata that occurs 

 as a plucked leaf withers. It is generally stated that marsh and 

 aquatic plants do not close their stomata under these circum- 

 stances. I find that, although the phenomenon is much less 

 marked than in terrestrial plants, yet that, in many species, 

 partial closure of the stomata undoubtedly occurs in the aquatic 

 class. 



The most interesting fact observed in withering leaves is that 

 in many cases the "closure" of the stoma is preceded by 



mporary opening, which may occur almost simultaneously 



iih the severance of the leaf from the plant. Thus the hygro- 

 .jiipe readings rise at first, and subsequently sink to zero. The 

 interest of this fact is the demonstration of the interaction 

 between the guard cells and the surrounding epidermis. The 

 jihenomenon is best seen in plants with milky juice, but is not 

 confined to this class. The preliminary opening of the stomata 

 occurs in the early morning, but not in the evening — a fact 

 which is of importance in relation to the mechanism of the 

 nocturnal closure of the stomata. 



A diminution of the stomatal transpiration can also be 

 lirought about by compressing the stem of the plant in a vice, a 

 process which is known to diminish the water supply (F. 

 Darwin and R. Phillips, Cainb. Phil. Soc. Proc, 1886). The 

 stomatal closure is here probably an adaptive response to the 

 lowering ot the water-supply of the leaf, but this is not quite 

 certain. 



A series of experiments were made on the comparative effect 

 of moist and dry air, from which it is clear that the stomata 

 " close " before any visible signs of flaccidity occur in the leaf. 

 When leaves are exposed to air dried by H2SO4, " closure " is 

 preceded by a remarkably prolonged opening of the stomata — 

 a phenomenon which requires further investigation. 



Baranetzky (Bot. Zeitung, 1872) showed that slight degrees of 

 disturbance affect transpiration. The hygroscope gives no 

 evidence of increased transpiration when the disturbance is 

 slight. When the plant is violently shaken the leaves become 

 flaccid and the stomata "close," and in some cases the closure 

 is preceded by increased transpiration, no doubt due to 

 temporary opening of the stomata, induced by the guard cells 

 being released from epidermal pressure before they have lost 

 their own turgor. 



N. J. C. Muller (Pringsheim's Jahrbiicher, vol. 8, 1872) 

 showed that stomata may be closed by electric stimulation ; my 

 experiments show that while a strong shock narrows the stomata, 

 a weaker one opens them, no doubt owing to the temporary loss 

 of epidermal pressure. 



Some experiments on poisonous gases and vapours were 

 made. Chloro.brm and ether slowly "close" the stomata, 

 which finally reopen in a normal atmosphere. Pure COj also 

 slowly closes the stomata. 



The hygroscope is well fitted to demonstrate the fundamental 

 facts in relation to light. The fact that the stomata are widely 

 open in sunshine is well known ; the difference between bright 

 and less bright diffused light is not so well known, nor the fact 

 that in dark stormy weather the stomata may be nearly closed 

 by day, even in summer. The effect of difference of illumination 

 is well shown in certain leaves having stomata in both surfaces, 

 e.g. Iris, Narcissus, and the phyllodes of Acacia cyclopis. In 

 these the stomata on the illuminated surfaces are much wider 

 i>pcn than on the less brightly illuminated sides ; and when the 

 plant is reversed in position in regard to light, the stomata 

 rapidly accommodate themselves to the change in illumination. 



The most interesting fact in regard to the effect of artificial 

 darkness is that it is more effectual in producing closure in the 

 afternoon than in the morning ; and, conversely, illumination 

 o|)cns 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 fact of interest is that in darkness 

 longed for several days the stomata gradually ojien. This 



t observation is used in the section on the mechanism of the 



ma as an argument against the prevalent view that the stoma 



■ses in darkness, because in the abeyance of assimilation the 



niotic material, on which the turgor of the guard cellsdepends, 

 t cases to be manufactured. 



Schellenberger {Bot. Zeitung, 1896) has striven to uphold 

 this view by showing that in the absence of COj the stomata 



1 I use the word " closure " to mean such a narrowing of the stomatal 

 .iperture as corresponds with zero on the hygroscope. 



NO. 1496, VOL, 58] 



close as though they were in darkness. My experiments on 

 plants deprived of CO.^ lead to absolutely contrary results, 

 namely, that the stomata remain perfectly open even during 

 prolonged deprivation of COo. 



It is a vexed question (Leitgeb, Mittheilimgen aus dem Bot., 

 Inst, zu Graz, 1886) whether or no the majority of plants close 

 their stomata at night. My conclusion is that in terrestrial 

 pl^ts (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. Stahl [Bot. Zeitung, 1897) 

 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 tempera- 

 tures. 



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 {Bieblatt zur Leopoldina, 1895) 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 com- 

 paratively 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 trans- 

 piring leaf is considerably cooler than the dry-bulb thermometer, 

 but at night it has almost the temperature 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 practical importance 

 to the plant I am inclined to believe, from Sachs' {Arbeit en, 

 1884) experiments on the depletion of leaves, that all saving of 

 heat must be valuable, by preventing the checking of trans- 

 location which he observed. 



The mechanism of the stoma is another subject which does- 

 not lend itself to condensed treatment. I have tried to point 

 out that the stoma has been neglected in the modern reorganisa- 

 tion of plant physiology from the point of view of irritability. 

 Some observers insist on the preponderant influence of the guard- 

 cells, while Leitgeb in the same way exaggerated the import- 

 ance of epidermic pressure, whereas the two factors should, as 

 far as possible, be considered as parts of a whole and as corre- 

 lated rather than opposed in action. I have also attempted to 

 show how the stoma, like other parts of the plant, may be 

 supposed to react adaptively to those signals, which we usually 

 call stimuli. The attempt which I have made to rank the 

 problem among the phenomena of irritability, is very tentative 

 in character. I have ventured to put it forth because I an* 

 convinced that it is in this direction that advances will be 

 made. 



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