IX. PRIMARY STEM STRUCTURE AND DEVELOPMENT Bot 404 Fall 2004 A. Shoot apex -plants have an open system of growth, therefore the ability (at least potentially) to continue growth because there is a meristem at the tip of each branch 1. Definitions -shoot apex = area above the base of youngest leaf; geographical term -promeristem = apical zone of undifferentiated cells; a meristem that gives rise directly to other meristems or to other, distinct parts of the same meristem; physiological term -both terms are useful; may coincide -other terms with no precise definition include: shoot tip, stem tip, growing point, vegetative tip 2. Evolution of apical meristems -in green algae (and other algae with multicellular bodies, e.g., reds, browns), no need to protect meristematic cells from drying out since plants are submerged -in terrestrial plants, the delicate meristematic cells or cells at the apex need protection from drying; two ways to do this a) by a mucilaginous covering secreted by glands; used by some terrestrial algae, some bryophytes, and some pteridophytes b) by a series of overlapping appendages that will form a dome over the apical meristem, enclosing it in a high humidity chamber (perhaps original function of leaves was to protect the apex, with photosynthesis a secondary function) 3. Patterns of shoot apex organization (DIAGRAM)
a) Apical mother cell -all cells in the shoot can be traced back to one cell -found in many algae, bryophytes, some pteridophytes b) Apical initials -all cells in the apex can be traced back to a group of cells -found in some pteridophytes (basically in all except for the ones with just one apical mother cell) c) Tunica-corpus -considerably more complex; originally limited to angiosperms (gymnosperms were defined as having cytohistological zonation) -now both gymnosperms and angiosperms are considered to have the same basic shoot apex organization -four zones are recognized: tunica, central mother cells, peripheral zone, and the pith-rib meristem; the latter three make up the corpus -tunica is usually uniseriate, but may have several to many layers; the cells usually divide only anticlinally, increasing surface area and giving rise to the protoderm; but if multiseriate, the inner layers may divide periclinally and contribute to the corpus -the central mother cells are analogous to the quiescent center of the root apical meristem; the cells are usually larger than those of other zones and cuboidal; cells in this zone divide more or less equally in all planes and make cells that go into the peripheral zone and pith-rib meristem -peripheral zone forms the cortex -pith-rib meristem forms the pith 4. Plastochron -plastochron = time between initiation of successive leaf primordia; also includes the changes in shoot apex size and shape during this period -during and just after leaf initiation, shoot apex is very small -as the next set of leaf primordia (or the next one) are arching over the apex, the apex is about mid-way in its expansion to greatest size -leaf primordium continues to grow, apex reaching maximum size and almost ready to produce a new leaf (or leaves), as indicated by the first few periclinal divisions that will produce the primordium(ia) -primordia are formed on shoot apex in a mathematically precise formula (phyllotaxy see Esau for details) -in monocots, leaf primordia typically attached 180 around the apex (wrapped around)
B. Primary stem structure 1. Basic structure in dicots (DIAGRAM) -basic pattern among plants is the same, variation is in the arrangement of vascular tisses -innermost layer of cortex may be an endodermis (suberized layer that keeps water from leaving the stele) 2. Concept of the stele -originally the term referred to the central core of the axis, including pith (if present), vascular tissue, interfascicular regions, and a bit of fundamental tissue on the periphery of the vascular system -current usage often refers only to the vascular system -vascular bundles--discrete strands of vascular tissue, usually xylem + phloem -why do xylem and phloem occur together? Not much work done on this. -phloem perhaps gets protection from being associated with the xylem or maybe mass flow works better with a ready water source 3. Stelar types a. Protostele vascular tissue is a solid core, with phloem surrounding xylem or the two are intermingled in strands/plates (don t need to know subtypes); no leaf gaps, no pith; found in pteridophytes including some ferns, roots of dicots and gymnosperms; common in fossil plants, the first type of stele to evolve b. Siphonostele in pteridophytes tubular stele (siphon = tube) with a pith (nonvascular core) present; leaf gaps may or may not be present; found in pteridophytes; 3 subtypes: i) ectophloic phloem only on the outside of the xylem; no leaf gaps ii) amphiphloic phloem on the inside and outside of xylem; leaf gaps present or not iii) dictyostele vascular tissue is dissected or broken up; phloem surrounds the xylem; interfascicular tissue indicates the leaf gaps
c. Siphonostele in seed plants -origin different from dictyostele of pteridophytes (non-homologous) -traces enter the leaves; leaf gaps are not homologous to those of pteridophytes a) eustele bundles arranged in a ring, with a pith, parenchymatous interfascicular regions in between the bundles; found in gymnosperms and dicots b) atactostele bundles are scattered (atacto = random, but the bundles are anything but random); may or may not be a pith; most common in moncots, but in a few dicots; considered most highly derived and complex stele 4. Bundle types a) collateral most common type in gymnosperms and angiosperms; general trend toward this type; phloem is toward the outside (abaxial side), xylem toward the inside; rare to find anything other than collateral bundles in the leaves of any plants b) bicollateral phloem on both sides of the xylem; in certain ferns and a few families of dicots [Apocynaceae (incl. Asclepiadaceae), Convolvulaceae, Cucurbitaceae, Myrtaceae, Solanaceae, certain tribes of Asteraceae]; become converted to collateral bundles when they enter the leaf c) amphiphloic (amphicribral) phloem surrounds the xylem; common in ferns; also becomes collateral in the leaf d) amphivasal xylem surrounds the phloem; most commonly found in ferns; also becomes collateral in the leaf e) isolated phloem fairly common to find isolated phloem connecting two collateral bundles; about 15 dicot families have extensive internal isolated phloem (e.g., Solanaceae); largely connected to floral and fruit structures, external phloem to leaves division of labor; examples of isolated xylem are unknown
C. Primary Stem Development 1. General pattern of primary differentiation -the promeristem leaves behind cells that can be identified as belonging to three primary meristems, which occur at different levels in the growing tip (DIAGRAM) -in the shoot apex, the corpus is transitional to the ground meristem and the tunica to the protoderm; the procambium forms a continuum from mature vascular tissue up to promeristem -lower down, internodes begin enlarging and elongating; this is where primary tissues are differentiating (e.g., protoxylem, protophloem, etc.) -in dicots and gymnosperms, can usually recognize regions of the fundamental tissue (cortex, etc.) but in monocots, simply called fundamental tissue -if secondary growth occurs, it is contributed partly by undifferentiated procambial cells and partly from fundamental tissues -axillary buds form from localized cell divisions (often having a concentric appearance); this is often called the shell zone (= cell divisions giving rise to an axillary bud); bud formation starts a few nodes down from the apex -leaf is the first major organ; axillary buds come later, so bud vascular system is attached to the leaf trace/leaf gap system after it is formed -axillary buds are usually under the control of the shoot apex, and their further development is usually suppressed by hormonal control -eventually the lower ones may be released from suppression; this often occurs if
the shoot apex is damaged, but it may happen eventually anyway -transfer cells associated with developing vascular traces of axillary bud 2. Longitudinal differentiation a) procambium continuous and acropetal b) protophloem continuous and acropetal c) protoxylem discontinuous and birdirectional; appears first as an isolated patch at the base of each leaf; develops and differentiates into leaf and down the stem; eventually connects with upwardly differentiating protoxylem from below d) metaphloem continuous and acropetal e) metaxylem discontinuous, bi-directional -logical that phloem should be acropetal and continuous because of the need for nutrient supply -why is xylem development discontinuous? Essentially the pattern is to construct sections of mature xylem and hook them up to an older functional part; no really good explanations available 3. Transverse differentiation a) proliferation of procambium until bundle is of a certain size (cells are elongating and dividing) b) protophloem appears at outer periphery of bundle c) protoxylem appears at inner periphery of bundle d) metaphloem appears at the inner edge of protophloem and differentiates toward the middle of the bundle e) metaxylem appears at the inner edge of protoxylem and differentiates toward the middle of the bundle f) in dicots and gymnosperms, there is leftover procambium in between the xylem and phloem that remains potentially meristematic -in monocots, all of procambium in a vascular bundle will differentiate into primary metaxylem and metaphloem, using up all potentially meristematic cells
4. Other primary meristems a) intercalary meristem -zone of meristematic cells that occurs just above the node or at base of leaf blade -common in monocots, rare in dicots -functions: increase in length, upright growth in lodged stems -increases flexibility but the immature tissue in the IM poses problems for photosynthetic transport and it produces a zone of weakness (hence the wrap-around sheathing leaf base) b) primary thickening meristem -allows stems without secondary growth to grow in diameter close to the apex -marked by presence of a thick disk of mitotic activity -allows production of a large number of nodes (and therefore leaves) while the plant is still close to the ground -occurs in bulbous plants and those with large, fleshy columnar stems (e.g., cycads, some dicots, many monocots) and probably also palms, bamboos