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 



