THE STEM 175 



at right angles to section A. This will give us the profile 

 of the pit, i.e. its section (C, D, E), which will explain 

 everything to us. It happens that the common walls 

 of the two adjoining cells form here a cavity shaped 

 like a lens, the margin of which corresponds to the 

 outer ring of the pit {b).^ This cavity seems to be 

 formed, as it were, by two watch-glasses, perforated in 

 the middle with round apertures {a in fig. A). These 

 apertures lie one below the other, so that their out- 

 lines blend into a single inner ring {a). In fig. B 

 both circles are seen, owing to the fact that we are 

 looking at them obliquely. The lens-shaped cavity 

 is divided into halves by a very thin membrane 

 stretched across it, the centre of which is thickened 

 like a disc (C) . The edges of this disc, generally visible 

 through the glass-like transparent cell-wall, give the 

 impression of the middle ring (c in A and B) . Having 

 realised the arrangement of these pits, and knowing 

 that the air and water in them are under tension like 

 that produced by a suction pump, their significance 

 is easily understood. They are valves — most perfect 

 valves. When the suction pressure in the vessels is not 

 great, water passes through the thin membrane (as 

 shown by the arrows at C). The resistance of these 

 membranes to the movement of water is insignificant : 

 if we let fall a drop of water upon the upper cut end of 

 a long branch, the exudation of a similar drop from 

 the lower cut end follows almost instantaneously. 

 But these thin membranes might easily break under 

 pressure such as we have observed in vessels. The 

 membrane then curves, and the disc applies itself to 

 the one or the other aperture according to the direc- 

 tion of the pressure (as indicated by the arrows at 

 D and E) . Thus the bordered pit is shown to be a very 

 conveniently arranged double valve, adapted to a variety 



1 Dotted lines are drawn to show the corresponding parts of all the 

 figures. 



