NATURE OF PLANTS 67 



ducts are characterized by reticulate or pitted walls since the 

 thickening largely occurs 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. 

 42, 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. 42, 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 disappear 

 although the rest of the protoplasm of the cells remains active 

 for one or more seasons. Elongated parenchyma cells are asso- 

 ciated with the sieve tubes and accompanying cells, serving 

 chiefly for the transport and the temporary storage of the more 

 readily diffusible carbohydrates. Fibrous cells, stereome, are 

 also of common occurrence in the phloem (Fig. 42, st). The 

 cambium is composed of very delicate cells, owing to the fact 

 that in this region cell division is taking place and the cells 

 consequently have not yet attained their characteristic form (Fig. 

 42, c). We may best gain an idea of the growth of the cambium 

 if we consider it as a single layer of cells between the xylem and 



