THE STEUCTUEE OF THE EOOT 27 



the portion (c) belo-w the overhanging margin of the secondary 

 membrane. This form of cell distinguishes the Conifers from 

 dicotyledonous plants, which have fully developed vessels. In 

 the latter, it often happens that in the summer the leaves 

 withdraw so much water from the vessels that the latter are 

 almost devoid of water and 

 filled with air. If this con- 

 dition of drought lasts for a 

 considerable time, the deli- 

 cate membranes will be in 

 danger of drying up, and thus 

 of losing their power of con- 

 ducting water. But in the 

 case of the funnel-like con- 

 striction of the bordered pits, 

 the water is retained in the 

 pit by capillarity, even when 

 the vessel itself is already 

 dry, and consequently the 

 danger of destroying the 

 functioning of the valve is 

 avoided. If afterwards one 

 cell becomes filled with 

 water while the other is still 

 devoid of wa;ter, a consi- 

 derable pressure is exerted 

 by the full cell ; this causes 

 the closing membrane to 

 bulge out towards the empty I'm- 9.— poktion of a thin longucdinal 



11 J , . ,. , Section THKOUOH THE Wood OE THE Pmk, 



cell. In doing tnis, tne showins bokdeked pits m suefaoe 



membrane, becoming ex- ^™^- 



tended, enlarges the area of "^. mediUlary-ray cells, showing the lens-shaped 

 ' ° sections of the bordered pits. 



passage. If a vessel becomes 



filled very suddenly and with great force (as, for instance, 

 if the air in one of the vessels becomes very much rarified), 

 the pressure can become so great that the closing membranes 

 are burst. In such a case the small thickening in the centre 

 lessens the danger, for as soon as the membrane is consider- 

 ably pushed in, the thickening presses against the wall of the 



