NATURE OF PLANTS 81 



vessels are now seen to be tubular structures with peculiar 

 thickenings of their inner walls that assume the form of rings 

 (annular vessels), spirals (spiral vessels), or the spirals may 

 branch more or less (reticulate vessels), and often to such an 

 extent that they cover the entire surface of the wall with the 

 exception of numerous small spaces, pores, thus forming the 

 pitted vessels (Fig. 43, '"). These peculiar sculpturings prevent 

 the crushing in and closing of these tubes and the annular and 

 spiral thickenings also permit considerable elongation of such 

 ducts which would not be possible if the thickening were more 

 uniform over the wall. It will be noticed that the later formed 

 ducts are characterized by reticulate or pitted walls since this type 

 of thickening must occur after the elongation has taken place. 

 The ducts are composed of elongated cells but owing to the 

 absorption of the majority of the cross walls the ducts finally 

 come to resemble hollow tubes that often run for considerable 

 distances through the stem without any cross walls at all. The 

 smaller cells of the xylem assume various shapes. Some of 

 them, the tracheids, have pointed ends and are characterized by 

 markings similar to the ducts. Strengthening fibers, similar to 

 those noted in the cortex, are of common occurrence and also 

 short cells with blunt ends, wood parenchyma. The ducts and 

 tracheids soon lose their cell contents but continue to function 

 in the conduction of water. The wood parenchyma and other 

 living cells retain their vitality for several years and generally 

 function as storage cells. It is not improbable that they may 

 furnish some of the energy required to force the water through 

 the ducts and tracheids. It is significant that living tissue is 

 always associated with these water-conducting cells of the stem. 

 Passing now to the phloem we see that the sieve tubes are made 

 up of elongated cells with perforated or sieve-like cross walls (Fig. 

 43, s). These minute openings adapt these cells to the transport 

 of albuminous substances which do not readily diffuse through 

 cell walls. The accompanying cells (Fig. 43, ac) which are cut 

 off from the sieve tubes probably assist them in this work. At 

 any rate they have abundant cell contents and contain nuclei, 

 whereas, singularly enough, the nuclei of the sieve tubes soon dis- 



