VESSELS 



35 



Raphia} Moreover, fibres treated in various ways are the 

 source of wood-pulp and other paper-making materials. 



The main function of vessels (Fig. 17) is water-conduction, 

 but they also are elements of mechanical importance on account 

 of their thick walls. These are necessitated by the frequent 

 existence of negative pressures in the water-conducting tracts. 



Fig. 17. — Vessels showing various types of thickening and perforation. 

 A, spiral ; B, annular ; C, portion of A in longitudinal section showing 

 attachment of thickening to wall ; D, end of vessel of Alder (Alniis) 

 showing ladder-like perforation ; E, vessel of Poplar (Populus) ; 

 F, vessel of Oak (Quercics) ; G, reticulate vessel of Marrow [Ciiciir- 

 bita). b., bordered pit ; p., perforations between segments of vessel. 



such as obtain, for instance, when transpiration is active. At 

 these times collapse of the vessels, under the positive pressure 

 exerted 'by the surrounding elements, is prevented by the rigidity 

 of their walls. The type of thickening varies greatly, depending 

 largety on whether the vessel is formed in a part of the plant 

 that is still gro\ring or in one that has reached maturity. In 

 the former case the thickening must be of such a nature as to 



' Regarding cotton see p. 103. 



