26 



THE PHYSIOLOGY OF PLANTS 



But suppose that, in the process of thickening, a small mass 

 of lime had fallen upon this portion of the sheet, and had 

 become hardened in the form of a small disk in the middle of 

 the separating membrane ; then if this mass is pushed towards 

 the other side, it, instead of the fist, would close the aperture. 

 In this somewhat rough illustration, the two halves of the 

 room represent two adjoining cells (Fig. 8), the sheet is the 

 -primary membrane (o), and the deposit of lime the secondary 

 thickening (s and s'), the funnel-shaped hollow towards the 

 other cell represents the widening of the bordered pit (c), the 

 portion of the sheet within the hollow the closing membrane 



of the pit, and the 

 small disk of lime on 

 the separating mem- 

 brane stands for the 

 thickening of the 

 closing membrane, 

 usually termed the 

 torus (t). 



Now, if the water 

 is at the same pres- 

 sure in two adjoin- 

 ing elements of the 

 tracheal system — for 

 instance, in two 

 tracheids — the clos- 

 membrane will 

 remain in its original 

 median position, and 

 if there should be only a small excess of pressure in either 

 cell, the water will easily pass through the thin portion 

 of the closing membrane, and the equilibrium will be readily 

 re-established. Such an easy functioning of the valve-like 

 arrangement we may assume to take place in such wood as 

 that of Conifers, which has no true vessels, but in which the 

 wood consists only of wood tracheids with a row of bordered 

 pits (Fig. 9). In this figure we see the pits in surface view, 

 and there the light central ring represents the free portion- of 

 the canal {t in Fig. 8), while the darker outer ring represents 



FiQ. 8.— Two Tkaoheibs seen in Tkansvkkse 

 Section. 



primary membrane ; 8 and s', secondary membrane ; 

 i, intercellular space ; t, thickening on the pit mem- 

 brane ; c, cavity of the bordered pit. 



