I'ilLOEM ELEMENTS 



171 



The cambium ceases its additions 

 of the bundles early in August, but 

 additions to the phloem until the 

 close of the growing season. 



As to the length of life of the 

 phloem elements, in some plants the 

 sieve tubes live but a single year 

 while in others they may survive and 

 remain functional for a few years at 

 most. The companion and phloem 

 parenchyma cells may die with the 

 sieve tubes, but in some cases they 

 survive these, even until they become 

 cut off from the general circulation 

 during the formation of borke (see 

 page 55). Therefore in stems several 

 years old we are apt to find the outer 

 and older portions of the phloem 

 collapsed and dead (Fig. 24). 



Relation of One Year's Phloem 

 Elements to those of the Next. 

 — Fig, 95 shows diagrammatically 

 how the phloem of one year narrows 

 down at the close of the year's 

 growth in length and lies in imme- 

 diate contact with the primary 

 (earliest-formed) phloem of the fol- 

 lowing year. The ending of the 

 year's phloem (where it tapers to a 

 point in the diagram) consists of 

 sieve tubes and companion cells in 

 Dicotyledons, and sieve tubes and 

 vertical rows of phloem parenchyma 

 cells in Gymnosperms; and these 

 join with the corresponding elements 

 that are first differentiated from the 



to the xylem or wood side 

 it may continue its slow 



Xylem 



Fig. 95. — Diagram to show the 

 relation of the food-conducting 

 tissues of the leaf to those of the 

 stem; and in the stem the relation of 

 these tissues of one year to those of 

 preceding years. The dotted line 

 between phloem and xylem stands 

 for the cambium. The figures at 

 the bottom of the diagram indicate 

 the age in years of the zones of 

 tissues in phloem and xylem. 



