226 THE MOVEMENT OF WATER THROUGH THE PLANT 



water-conducting cells and vessels (Fig. 6o, J). Simple pits, on the other 

 hand, are restricted almost exclusively to the walls of living cells (Fig. 6o, B). 

 The stages in the development of a vessel, which is a more complex phe- 

 nomenon than the formation of the other elements of the wood, are indicated 

 in Fig. 6l. The original cell resulting from division of a cambium cell in- 

 creases rapidly in diameter, simultaneously developing a prominent vacuole. 

 Lignification of the lateral walls of the cell takes place at about the same 

 time that dissolution of the end walls of the vessel segment occurs. Disinte- 

 gration of the protoplasm also occurs at this stage in vessel development. The 

 result of this series of processes is the formation of a typical tubular, non-liv- 

 ing vessel by the coalescence of a number of vessel segments each of which 



A B 



Fig. 6o. Diagrammatic representation of {A) bordered pit, sectional (above) and 

 face views, (B) simple pit, sectional (above) and face views. Redrawn from Eames 

 and MacDaniels (1925). 



has been differentiated from a single cell originating from a division of a 

 cambium cell. In many species some of the cross walls between vessel seg- 

 ments fail to disintegrate. Such cross walls are almost invariably perforated 

 by one or more openings (Fig. 57). The distance between the residual cross 

 walls in xylem ducts varies greatly from species to species and often from 

 vessel to vessel in the same individual plant. There is some evidence that the 

 distance between the cross walls in the vessels of ring porous species is usually 

 greater than in diffuse porous species (Handley, 1936). According to Priestley 

 (1935), in some species of trees at least, disintegration of the end walls occurs 

 almost simultaneously between all of the newly differentiated vessel segments 

 which are present as a continuous chain of cells from the top to the bottom 

 of the tree. 



