IMPORTANCE OF STOMATA. 25 1 



and prevents the filtration of the water from the guard-cells into the epidermis, 

 without, however, interfering with the filling of the cells by endosmose ; in the dark, 

 on the contrary, or even as the intensity of the light diminishes, the protoplasmic 

 utricle would, according to my assumption, become more permeable and less 

 resistant to filtration, and in consequence of this a portion of its water might be 

 pressed out from the guard-cells, previously forcibly distended, through the wall e. 

 That such an irritability is not opposed to the phenomena of irritability elsewhere 

 existing in the vegetable world will be sufficiently demonstrated by subsequent 

 lectures on the irritability of plants. 



To any one ignorant of the facts, the theme here treated of may appear somewhat 

 small and insignificant, on considering how extremely minute the stomata are, and 

 that the diameter even in the open condition amounts to only a few hundredths 

 of a millimetre. Through such extremely fine openings (of which we may ob- 

 tain an idea by drawing out glass tubes to hair-like fineness) only extremely little 

 water can evaporate or other gas pass even in a long time. But the fact 

 is altered when we reflect how extremely numerous the stomata may be on 

 the assimilating green foliage-leaves — one hundred to two hundred on a square 

 millimetre of the epidermis are common, and they are not rarely much more 

 numerous; so that a leaf say the size of the hand possesses many millions of such 

 fine openings, which simultaneously open when the sun shines on the leaf, and 

 simultaneously close when it becomes shaded. The alteration in the discharge of 

 the aqueous vapour developed in the intercellular spaces, effected by the opening ' 

 and closing under varying conditions of illumination, is enhanced by the warming 

 of the tissues of the leaf, which usually accompanies intense illumination, so that 

 an increased formation of vapour then takes place. Consequently the tension of 

 the vapour is raised, and it is forcibly expelled from the open apertures. In the 

 shade and in darkness, on the other hand, the formation of vapour in the interior 

 diminishes as the temperature falls, and the stomata being closed, the vapour at 

 low tension is retained ; whence complete saturation of the intercellular spaces with 

 vapour must soon occur, preventing the further formation of vapour, especially during 

 the sinking of the temperature at night. It scarcely needs mention that all these con- 

 siderations can only refer to the stomata on the organs of transpiration ; and that, 

 self-evidently, those occurring on subterranean shoots and on parasites containing 

 chlorophyll, only come into consideration in so far as they present, since they exist, 

 open paths of communication for an extremely slow exchange of gases, and that we 

 have here to do with organs of modified or even entirely suppressed function. 



Besides the varying illumination, and the consequent opening and closing of 

 the stomata, account has to be taken of many other factors, by means of which 

 transpiration may be accelerated or diminished. In the first place, so much is 

 clear, that even when the stomata are open, the escape of aqueous vapour from 

 them must be less rapid in proportion as the surrounding air is already laden 

 with vapour. In other words, transpiration must be diminished by the moisture of 

 the surrounding air; it is entirely prevented, however, only when the air is com- 

 pletely saturated, and the temperature of the leaves is not higher than that of the 

 surrounding air. Were the latter the case, the acjueous vapour, expanded even more 

 by the higher temperature, could still escape. Since, further, the formation of vapour 



