CHEMICAL AND PHYSIOLOGICAL STUDIES. 



175 



— is put into action, and during the da3'time, for the most part, 

 water is evaporated from the leaves, and as constantly replaced 

 by raw sap drawn up from the roots, and, ultimately, the soil. 



In direct sunlight and in hot dry weather, the transpiration 

 current is much more rapid than in diffuse daylight, or colder 

 weather; also, the relative humidity of the atmosphere influences 

 the rate of evaporation of water from the leaves, so that, in 

 moist weather, the current may be greatly diminished. 



With regard to the elements of the xylem ^hich function 

 most in this upward conduction of sap, it has been found that 

 the tracheides exercise by far the greatest influence. At times 

 air -bubbles in the tracheides may act, either by leasoii of 

 capillarity, or a sort of pumping action, as distinct aids to the 

 flow. It was formerly thought that the cell-walls formed the 

 most important 

 paths of self-con- 

 duction ; but al- 

 though the walls, 

 when saturated, 

 may form conduct- 

 ing strands, the 

 sap passes mostlj' 

 by way of the 



cavities of the -p^g 113.— Drawing showing the Mode ofTeemina- 

 tracheides. In tion of the Vascular Uundles in the Meso- 



JPHYLL OF A Bifacial Leaf (Begonia). — a, Spiral 

 and annular vessels ; /i, phloem portion of tlie 

 bundle ; 6, cells of the mesophyll. 



trees the only 

 wood which con- 

 ducts sap is the 

 alburnum or young wood; the heart-wood or duramen is always dry. 



In tracheides with bordered-pits, the flow of sap may at 

 times be prevented from passing in the transverse direction by 

 reason of the forcing of the torus on to the lumen of either side 

 of the pit. 



That the transpiration, current is not the only factor at work 

 in producing the upward flow of sap in a plant is shown by 

 cutting across a stem below all the leafy parts, when it will be 

 found that sap constantly exudes through the cut xylem; by 

 connecting up the cut surface with a manometer (a mercurial 

 pressure-gauge) the force producing this exudation may be 

 measured, and is sometimes found to be considerable. This 



