116 VEGETABLE PHYSIOLOGY 
The external conditions to which a plant is exposed 
have a considerable influence upon the gaseous currents, 
The effect of light upon a green plant has already been 
alluded to. The influence which it exerts is an indirect 
one, affecting the consumption of carbon dioxide and the 
liberation of oxygen. Nearly all the vital processes are 
subject to modification by the various external conditions. 
Transpiration we have seen to be very largely influenced 
thereby, and the varying amounts of watery vapour exhaled 
introduce variations in the amounts of the purely gaseous 
interchanges. The influence which the variation of the 
quantity of water in the plant exercises takes the form 
especially of modifying the width of the stomatal apertures, 
and hence of favouring or checking the entry and exit of 
gases into and from the leaves. 
Mechanical disturbances due to wind are of some 
importance, generally increasing the gaseous interchanges. 
Diminution of the turgidity of the tissues, amounting 
sometimes to flaccidity, interferes at times to a serious 
extent, the intercellular spaces becoming narrowed by the 
falling together of the cell-walls, a phenomenon which is 
noticeable also in the partial or complete closure of the 
stomatal orifices, due to the flaccidity of their guard-cells. 
Variations of barometric pressure and of temperature 
also influence to a considerable extent the process of diffu- 
sion within the plant, as well as the interchange between 
the interior and the external air. 
The movements of the air in the plant are subject to 
disturbance also by the setting up of the negative pressure 
in the cavities of the vessels of the wood which we have 
seen to be caused by active transpiration. This negative 
pressure can be demonstrated with considerable ease in the 
cases of woody stems, but it can be seen also in plants in 
which the development of wood is only very slight, having 
been observed in some cases in the elements of the central 
cylinder of some of the stouter Mosses. 
To demonstrate the existence of the negative pressure 
