. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. ay survive andremain functional for a few years atmost. The companion and phloemparenchyma cells may die with thesieve tubes, but in some cases theysurvive these, even until they becomecut off from the general circulationduring the formation of borke (seepage 55). Therefore in stems severalyears old we are apt to find the outerand older portions of the phloemcollapsed and dead (Fig. 24). Relation of One Years PhloemElements to those of the Next.—Fig. 95 shows diagrammaticallyhow
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. ay survive andremain functional for a few years atmost. The companion and phloemparenchyma cells may die with thesieve tubes, but in some cases theysurvive these, even until they becomecut off from the general circulationduring the formation of borke (seepage 55). Therefore in stems severalyears old we are apt to find the outerand older portions of the phloemcollapsed and dead (Fig. 24). Relation of One Years PhloemElements to those of the Next.—Fig. 95 shows diagrammaticallyhow the phloem of one year narrowsdown at the close of the yearsgrowth in length and lies in imme-diate contact with the primary(earliest-formed) phloem of the fol-lowing year. The ending of theyears phloem (where it tapers to apoint in the diagram) consists ofsieve tubes and companion cells inDicotyledons, and sieve tubes andvertical rows of phloem parenchymacells in Gymnosperms; and thesejoin with the corresponding elementsthat are first differentiated from the to the xylem or wood sideit may continue its slow. Xylem Phloem FIG. 95. — Diagram to show therelation of the food-conductingtissues of the leaf to those of thestem; and in the stem the relation ofthese tissues of one year to those ofpreceding years. The dotted linebetween phloem and xylem standsfor the cambium. The figures atthe bottom of the diagram indicatethe age in years of the zones oftissues in phloem and xylem. 172 TRANSPORT OF FOODS procambium of the succeeding years growth in length. It willbe seen by this diagram that the food highways in the leaveshave direct communication with the primary phloem in the newsegment of stem which bears them, and this primary phloem isin turn continuous with the phloem elements which the cambiumbuilds the current season throughout branch, trunk and roots. As has been said, in some plants the sieve tubes are functionalfor one year only; in others for two or more years, and it w
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