88 BOTANY OF THE LIVING PLANT 



water thus absorbed. This apparatus may be used to show how the 

 amount absorbed depends upon external conditions. Thus the 

 shoot may be covered by a glass bell, which will have the effect of 

 diminishing the amount; or it may be placed in a draught of air, 

 which will increase it. Or observations may be made at different 

 temperatures, and under different conditions of lighting, so as to test 

 the effect of these also upon the amount taken in to make up for loss 

 by transpiration. 



The strength of the suctional influence may be demonstrated by 

 fitting a cut shoot by an air-tight joint into a glass tube filled with 

 water, with its lower end dipping into mercury. The suctional 

 influence, absorbing water from the column in the tube, will raise the 

 mercury to a height in some cases equal to a suction of one atmosphere. 

 Thus suction from above is a very active factor in the movement of 

 the Transpiration-Stream. 



If suction from above be transmitted from the transpiring cells to the 

 cavities of the tracheides and vessels, through which the supply is conveyed, 

 this at once directs attention to the nature of their contents, and to the pres- 

 sures to which they are exposed. The vessels and tracheides are full of fluid 

 when young. It has been shown by Dixon and Joly that an unbroken column 

 of water in a closed tube, with its walls wetted by it, has such cohesion that 

 it can transmit a tension like a rigid solid. By such unbroken columns of 

 fluid in the young vessels and tracheides the suctional influence could then 

 be transmitted from above throughout the conducting tract. Moreover, it 

 has long been known that the contents of the vessels are normally at less than 

 the atmospheric pressure, and an indication of this is seen by the absorption 

 of water at the surface of a freshly cut stem. These facts support the con- 

 clusion that the suctional tension from above is transmitted downwards 

 through the tracheae. 



But as vessels grow old, bubbles of gas may frequently be seen in their 

 contents, which would appear to break the fluid columns. The individual 

 vessels are, however, continuous as open tubes only for a short distance, and 

 tracheides are even shorter. Each is finally closed by a water-saturated 

 wall, which is impervious to gas-bubbles. Hence these bubbles of gas would 

 only expand, under reduced pressure, till they completely fill the tracheae in 

 which they occur. The water-column of the stem would only be completely 

 interrupted if the tracheae containing bubbles were to form in some place an 

 unbroken diaphragm across the conducting tissues of the stem. Provided 

 a few were still full of fluid, that would suffice for the transmission of the 

 suctional tension downwards. 



A difficulty may be expected to arise in the case of tall trees. But Dixon 

 sums up the case for them thus : " The transpiring cells of the mesophyll 

 normally remain turgid during transpiration ; accordingly we would expect 

 that in high trees the osmotic pressure keeping them distended must correspond 

 in magnitude to the tensions necessary to raise the sap. This surmise has 



