In the early stages of devpt of the embryo, ALL THE CELLS undergo divisionBut with further growth and development, CELL DIVISION AND MULTIPLICATION become restrictedTo special parts of the plant
Comments that make histogen theory not acceptable today
The vegetative shoot apex is a dynamic structure thatin addition to adding cells to the primary plant body,repetitively produces units, or modules, called phytomeres
Apical meristem and derivative regions in roots. A, B, horsetail (Equisetum). A single apical cell (black triangle) isthe source of all parts of the root and rootcap. Heavier lines in B outline merophyte boundaries. The innermostboundaries of older merophytes are diffi cult to determine. C, D, spruce (Picea). All regions of root arise from onegroup of initials. The rootcap has a central columella of transversely dividing cells. The columella also gives offderivatives laterally. E, F, radish (Raphanus). Three layers of initials. The epidermis has common origin with therootcap and becomes delimited on the sides of the root by periclinal walls (arrows in F). G, H, grass (Stipa). Threelayers of initials, those of rootcap forming a calyptrogen. The epidermis has common origin with the cortex. (B, afterGifford, 1993; C–H, from Esau, 1977.)
Magnification of the root apical meristem of a fern. The single apical cell is very prominent in this micrograph, or at least its nucleus is. Notice the thin, flat cell between the two arrows: it is a progeny cell of the apical cell, but it was cut off to the forward side of the apical cell. This flat cell will continue to divide, and its progeny cells will become root cap cells. The cells cut off from the other three faces of the apical cell (only two of those three faces are visible here) will all contribute to the root proper. You can even see that these progeny cells divide so as to produce sets of brick-shaped cells that lie parallel to each other.
Magnification of the center of a root apical meristem of cattail. Even at the very center of the apical meristem, it is possible to distinguish clearly between the root proper and the root cap (arrows). In addition, the cells that give rise to the root cortex (the cortical initials) can be traced to a distinct set of cells just distal to those (the central cylinder meristem) that are producing the vascular tissues (the stele). In the root cap, the columella mother cells produce the central cells (the columella) of the root cap, and those cells in turn produce cells that make up the lateral parts of the root cap.
Longitudinal section of a root apex in corn (Zeamays). This longitudinal section shows many of the same features as occur in cattail (Typha) in Fig. 6.8-2. Here too the root cap is distinct from the root, and the central cylinder meristem is producing a broad set of cells that will develop into vascular tissues. Notice how wide the entire root is, even here near the apical meristem. You may be able to identify more than 50 rows of cells, so the mature root will be at least that many cells wide. That is quite broad, and is much more typical of roots of monocots rather than dicots. Dicot root apices tend to be much smaller. A higher magnification is shown in Fig. 6.8-4b.
Distribution of growth regions in a culm of rye plant. The plant has five internodes and a spike. The leaf sheaths arerepresented as extending upward from each node and terminating where leaf blades (shown only in part) divergefrom them. The youngest tissue in internodes (intercalary meristems) is represented in black, somewhat older tissuesis hatched, and the most mature is left white. Curves to the right indicate mechanical resistance of internodal tissues(solid lines) and of sheaths (broken lines) at the various levels of shoot. Resistance was equated with the pressure,expressed in grams, necessary to make a transverse cut through the internode or sheath.
DIFFERENTIATION<br />Process of growth and morpho-physiological specialization of the cells<br />
STAGES Of development of Primary meristems<br />Promeristem = apical initials + derivatives<br />Initial = cells which remain within the meristem<br />Partly differentiatiatedmeristematic zone:<br />Protoderm<br />Procambium<br />Ground meristem<br />
A. Shoot apex<br />Shoot apex- where new leaves and tissues of the stem arise<br />Models of tissue organization in the shoot apex<br /> A. Apical cell theory<br /> B. Histogen theory<br /> C. Tunica-corpus (most accepted in angiosperms)<br />
Apical cell theory<br />*Pteridophytes- apical cell (1 initial) or apical initials– tetrahedral (pyramidal), base is directed towards the surface of the apex<br /> *Gymnosperms- surface meristem; (apical initials– periclinal)<br /> central mother cells<br />
Histogen theory (Hanstein, 1868)<br />Histogen theory (Hanstein, 1868)<br /> 1. dermatogen – outermost<br /> 2. plerome – central<br /> 3. periblem – between 1 and 2<br />Each develop from independent group of initials (histogens)<br />Meristems are destined from the beginning to produce certain tissues<br />
Comments <br />All cells have basically equal potential of differentiation <br />One zone of apical meristem may contribute cells to another one<br />
Tunica- corpus theory (Schmidt, 1924)<br />Two regions: TUNICA and CORPUS<br />No constant relationship can be traced between the particular initials of the promeristem and the inner tissues of the shoot<br />2 regions can be distinguished by their plane of cell division<br />
Zonation in shoot apex<br />Central zone– (waiting meristem)- promeristem<br /> - corpus + portions of tunica <br /> - gives rise to:<br />Rib zone or pith rib meristem<br /> - below central zone; center location<br /> - becomes the pith<br />Peripheral zone or peripheral meristem<br /> - encircles the other zones<br /> - most meristematic (eumeristem)<br /> - densest protoplast and smallest dimensions<br /> - gives rise to leaf primordia,procambium, cortical ground tissue<br />
Origin of leaves<br />Initiated by periclinal divisions at the side of the apical meristem<br />Origin: tunica or corpus<br />Division leaf buttress<br />Affects Periodic changes in shape of shoot apex<br />BRANCHING<br />Where do branches originate?<br />Superficial layers --- exogenous<br />Axillary buds<br />
B. Root apex<br />Bi-directional production of cells<br />Subterminal in position<br />No lateral appendages (leaves, branches)<br />Branches occur beyond region of most active growth<br />Endogenous branching<br />Grows uniformly (no nodes and internodes)<br />
Root apex<br />Protoderm<br />Meristem of the cortex<br />Meristem of the vascular cylinder<br />Promeristem<br />columella<br />CLOSED TYPE- the initials are already discrete immediately adjacent to the central cells<br />Calyptrogen- intials of the root cap<br />OPEN TYPE – tissue systems become distinct only some distance away from the central cells<br />
Root apices<br />Single apical cell or initials (vascular cryptogams)<br /> Angiosperms : CLOSED and OPEN type (~ Histogen theory)<br />a. CLOSED – 3 tiers or layers of initials<br /> - apex of central cylinder<br /> - cortex<br /> - root cap<br />
b. Open type<br />- without any boundaries with reference to the derivative regions of the root<br />
Quiescent center- low mitotic activity<br /> - reservoir of cells<br /> - may be due to hormones ( high levels of auxin) , pressure exerted by rapidly dividing neighbouring cells (antagonistic direction of cell growth)<br />
Intercalary meristems<br />Isolated meristematic regions that are disjunct from the subapicalmeristematic region<br />Inserted between differentiated tissue regions<br />Internodes mature basipetally<br />Nodes mature first<br />Stems of monocots: internodes and leaf sheaths; in Equisetum<br />
1. Storied or stratified cambium<br /> -- fusiform initials are arranged in horizontal rows so that their ends are at the same level<br />2. Non- storied<br /> -- fusiform initials partially overlap one another<br />
Cork cambium<br />Phellogen<br />One type of initials<br />Rectangular in xs; regular polygons in ls<br />Vacuolated; may have chloroplasts and tannins<br />No intercellular spaces<br />Part of the periderm<br />Origin: external to VC– epidermis, cortex, phloem parenchyma<br />