EFFECTS OF EXTERNAL STIMULI UPON STOMATA 459 



stomatic pore depends entirely upon the internal tension in the guard- 

 cells, only those external factors which affect this internal tension will 

 induce movements of the stomata. As a matter of fact, it was dis- 

 covered long ago by Von Mohl, that the stomata of foliage-leaves open 

 when they are illuminated, and that this movement takes place most 

 rapidly in direct sunlight. Schwendener not only confirmed Von 

 Mohl's observations, but also showed that the stomata close when a 

 leaf is darkened. 



The opening of stomata under the influence of light is readily 

 comprehensible from an ecological point of view. Under normal con- 

 ditions, it is to the interest of photosynthesis that the stomata should 

 open as widely as possible, when the leaf is illuminated, so long as there 

 is no danger of excessive transpiration ; a high rate of transpiration is 

 in itself advantageous, within certain limits, because of the resulting 

 increase in the supply of mineral salts to the leaves. It is not so clear 

 what advantage is derived from the closure of stomata which takes 

 place at night, or when a leaf is otherwise darkened. F. Darwin 

 suggests, that this restriction of nocturnal transpiration serves to 

 prevent the serious loss of heat which would result, if evaporation were 

 as active at night as it is by day. Leitgeb, Stahl and Darwin all note 

 that a number of plants, notably marsh- and water-plants, and also 

 species with nyctitropic leaves, never close their stomata at night. 



Opinions differ considerably as to the influence of temperature 

 upon the movements of stomata. N. J. C. Miiller maintains that a rise 

 of temperature always causes the stomata to open, but Schwendener 

 disagrees with this conclusion. Leitgeb states that the stomata of the 

 pedicels and perianth segments of Galtonia candicans tend to open with 

 rise of temperature, whereas those on the foliage-leaves of the same 

 plant exhibit no such tendency. Kohl asserts that all stomata open 

 more or less rapidly, not only when the surrounding air is warmed, but 

 also when they are exposed to the action of (invisible) thermal radiations. 



The majority of stomata are exceedingly sensitive to any fluctua- 

 tions in the humidity of the atmosphere, and in the water-content of 

 the soil ; it is this property, above all others, that enables these organs 

 to exercise a beneficial control over the rate of transpiration. While 

 Von Mohl and other even earlier observers were aware of the fact, that 

 the stomata close as soon as a leaf begins to wilt, it was Leitgeb who 

 first pointed out that the closing movement commences, before any other 

 symptoms of withering make their appearance. If a leaf is transferred 

 from the moist atmosphere of a greenhouse to an ordinary room, its 

 stomata close even in bright daylight, a circumstance which proves that 

 the relative humidity of the atmosphere has a more powerful effect 

 than light upon the turgidity of the guard-cells. The ecological 



