54 STRUCTURAL BOTANY 



of subsequent growth in length, for as the vessel 

 becomes longer the rings become pulled farther and 

 farther apart. This form is not so very common in 

 the Wallflower. These innermost vessels constitute 

 the protoxylem, so called because this part of the 

 wood is formed first, and is therefore the oldest (see 

 Fig. 22,pxy). 



The vessels a little farther towards the outside are 

 still spirally thickened, but here the coils of the 

 spiral are closer together, for the thickening has been 

 developed a little later, and so has not got so much 

 pulled out by subsequent growth. 



Next we come to vessels in which the thickening 

 on the walls forms a network (reticulated vessels), the 

 meshes of which are occupied by a thin membrane. 

 This form is not capable of longitudinal extension, 

 and is only developed after growth of the stem in 

 length is complete. 



Lastly, in the outer and later-formed parts of the 

 xylem, we find vessels in which the wall generally is 

 thickened, but a number of little oval spaces are left 

 thin. These vessels are said to be pitted, the thin 

 parts of the wall being the pits. The thin places or 

 pits serve for the passage of liquids from one vessel to 

 another; the various kinds of thickening have the 

 object of stiffening the vessel and enabling it to resist 

 pressure. 



We have seen that the vessels are open, continuous 

 tubes. They are therefore quite different from 

 ordinary cells such as we described above. A vessel 

 is not a cell, but it is formed from a number of cells. 



