DEVELOPMENT OF THE VASCULAR SYSTEM IN THE LEAF OF BARLEY (HORDEUM-VULGARE L)

The development of the vascular system in the leaf of barley, Hordeum vulgare L., was studied from its inception to maturity. Young primordi a were examined with the scanning electron microscope, and distinctive early morphological stages of primordium development were correlated with the initiation of the longitudinal procambial strands. All longit udinal procambial strands originate in isolation, without continuity w ith the previously existing stem vasculature. The first-formed strands , which have large-bundle anatomy in both blade and sheath of the matu re leaf, arise in the disk of insertion (the stem segment associated w ith the primordium) and from there develop acropetally into the free p ortion of the primordium and basipetally into the stem below. Subseque ntly formed strands arise in the primordium. These strands are interpo lated between older strands across the width of the primordium in a pr ecise order, and they appear to develop both acropetally and basipetal ly. By the time the young leaf is 4 mm long, all of its longitudinal p rocambial strands have been initiated; in addition, the ligule has bee n initiated, delimiting blade from sheath. The first protophloem and p rotoxylem elements of the large strands arise in the disk of insertion near the site of initiation of each strand, the protophloem in advanc e of the protoxylem. From the site of initiation, the protophloem and protoxylem develop acropetally into the primordium and basipetally int o the stem, where, at a later stage of development, they interconnect with phloem and xylem in the nodal region of the leaf two nodes below. During further leaf growth, three successive regions can be identifie d within the blade: (1) a distal, mature region in which elongation ha s ceased and metaphloem and metaxylem elements are differentiating bas ipetally in all strands, (2) a region in which elongation is occurring and protophloem and protoxylem elements are mature or nearly mature b ut in which metaphloem and metaxylem elements have not yet begun to di fferentiate, (3) a region in which cell division is occurring and some protophloem and protoxylem elements are still differentiating.