Dr. T logo BIO 410/510 Plant Anatomy
Apical Meristems

Patterns of growth

•  Diffuse growth

•  Growth takes place throughout the tissues of the organism.

•  Common for filamentous algae and fungi.

•  Animals have diffuse growth

•  Localized growth

•  Growth is restricted to special regions—meristems.

•  Vascular plants have localized growth.

•  This means that a plant consists of:

•  1. Meristems - perpetually juvenile cells

•  As meristems divide, some daughter cells remain meristematic, thus perpetuating the meristem, while others start down pathways of differentiation into mature tissue cells.

•  2. Regions of growth and differentiation

•  In these regions, cells enlarge and undergo changes that lead to the characteristics that are appropriate for the kind of cells that mature from that region.
•  Cells in these regions are still considered meristematic, but they have started down a particular developmental pathway that ends with a particular tissue or tissue system (e.g. protoderm, provascular tissue).

•  3. Mature tissues

•  When a tissue is mature, there are no further cell divisions and growth ceases.

•  Advantage - developmental plasticity

•  New growth can be modified depending on current conditions

•  For example

•  A stem is blown over by the wind.
•  Meristems can produce new vertical branches.

•  Disadvantage - meristems are sensitive to damage.

•  Plants solve this problem by have "reserve meristems" (e.g. axillary buds) that remain dormant in reserve in case needed.

•  Totipotency

•  All cells in an organism have a complete genome

•  Every cell has the potential to produce an entire organism.

•  Can be demonstrated by growing tissue cultures

•  Tissue culture has been done starting with parenchyma, collenchyma, procambium, epidermis, vascular cambium, microspore mother cells

•  Cells grow first into callus (undifferentiated tissue)

•  If conditions are right, callus will develop meristems, which in turn will produce mature tissues, even a whole plant.

•  This method is even used for plant propagation.

•  All plants produced from the same source cells will be a clone.

•  Meristematic tissue is not "undifferentiated."

•  The old idea that meristems are made up of undifferentiated tissue is wrong.

•  Callus tissue is truly undifferentiated because it is a mass of cells all similar in basic structure.

•  But meristems are highly specialized—specialized for the function of cell division producing specific patterns of new tissues in an ordered fashion.

•  Note that before callus differentiates into a new plant, meristems must first differentiate from the callus.

•  Indeterminate growth

•  Plants have the potential to grow for an unlimited time producing new organs and tissues as long as meristems remain viable.

•  Size of a plant is determined more strongly by the environment than the size of an animal is.

•  Certain organs (stems, roots) have this potential of unlimited growth.

•  These organs can continue to grow as long as their apical meristems remain active. Meristems do not get "used up."

•  When meristem initial cells divide, on average, one of the two daughter cells remains meristematic, while the other produces more cells that eventually differentiate.

•  Determinate growth

•  Some organs, leaves for example, do not grow indefinitely. These are determinate organs.

•  Determinate organs reach a maximum size then stop growing.

•  Determinate organs have a finite lifetime but can be replaced by new organs by other meristems.

•  Sometimes a whole plant can be thought of as "determinate."

•  These plants live for one or two seasons, reproduce, then die.

•  Annuals
•  Biennials

•  Although these plants are "determinate," they still have organs with indeterminate growth (while they are alive).

•  Perennials can be thought of as indeterminate plants.

Meristems and meristematic regions

•  A meristematic region always contains a meristem which establishes patterns of growth for the meristematic region.

•  Often the meristem itself is the least mitotically active area of the meristematic region, but its divisions do set the patterns of growth.

•  The meristem acts as a reservoir of "genetically sound" cells

•  Since these cells have done relatively few mitotic cycles, they are less likely to have accumulated mutations.

•  The cells beyond the meristem divide and enlarge rapidly. Mutations in these regions are not as detrimental because they are active for a relatively short time, and mutations may affect genes not involved with differentiation of the tissues that a specific group of cells gives rise to.

•  (Copying record to tape example)

Meristem types

•  Apical

•  Located at the tip of an organ

•  Shoot apical meristem produces all primary tissues of a shoot branch.

•  Root apical meristem produces all primary tissues of a root branch.

•  Basal

•  Located at the base of an organ

•  Not common in vascular plants, but sometimes found in large leaves that have lengthy growth period.

•  Intercalary

•  Located between two regions of mature tissues

•  There are mature tissues both above and below the meristem.

•  Sometimes found at nodes

•  Lateral

•  Cylindrical shaped meristems, usually one cell thick

•  Responsible for secondary growth in plants that have it.

•  Vascular cambium

•  Produces secondary vascular tissue

•  Located between xylem and phloem

•  Produces secondary xylem to the inside, secondary phloem to the outside.

•  Cork cambium

•  Produces secondary dermal tissue (periderm)

•  Axillary

•  Located in the axil of a leaf

•  Responsible for branches of the shoot

•  May remain dormant as an axillary bud for extended time.

•  When active, it becomes the apical meristem of the branch that it produces.

The shoot apical meristem

•  Shoot apical meristem

•  Apical cell type

•  Common in ferns and other seedless vascular plants

•  There is a single initial called the apical cell.

Apical cell type
Apical cell type shoot apical meristem. Common in seedless vascular plants. There is a single initial cell from which all primary growth is derived.

•  All primary tissues can be traced back to the apical cell.

•  The apical cell is shaped like an inverted pyramid.

•  The apical cell divides parallel to its downward-facing sides, producing derivatives that divide further.

•  Tunica-corpus type

•  The characteristic meristem type of angiosperms

•  There are two zones of initials, tunica and corpus.

Tunica corpus meristem
Tunica-corpus type shoot apical meristem, characteristic of angiosperms. Surface layers are derived from the tunica; other tissue layers from corpus.

•  Tunica

•  Consist of the surface layers.
•  Can be 1-several tunica layers.
•  Tunica layers remain distinct because their cells divide only anticlinally.
•  Outer tunica layer gives rise to the protoderm.
•  Inner tunica layers contribute to peripheral zone.
•  Monocots typically have one tunica layer.
•  Dicots typically have two tunica layers.
•  Number of tunica layers often increase when a vegetative meristem transforms to a floral meristem.

•  Corpus

•  The core of the initial region below the tunica layers.
•  Cells divide in all planes
•  Contribute to peripheral zone and rib meristem.

•  Using chimeras to distinguish tunica and corpus

•  Other shoot meristem types

•  Gymnosperms typically have a group of initials, but not organized into tunica and corpus.

Phyllotaxy (patterns of leaf arrangement)

•  Basic leaf arrangement is based on the number of leaves per node.

•  Spiral (alternate). One leaf per node.

•  Opposite. Two leaves per node

•  Distichous (2-ranked). Leaves oriented in one plane

•  Decussate. Each pair of leaves is oriented at right angles to the pair at the next node.

•  Whorled. More than 2 leaves per node

•  Parastichies and the Fibonacci series

•  For spiral phyllotaxy, various spirals can be traced from younger to older leaves (or leaf primordia).

•  These spirals are known as parastichies.

•  One such spiral would start at the youngest leaf and trace through successively older leaves.

•  The "tightness" of this spiral can be defined by:

•  The number of turns around the stem and

•  How many leaves the spiral passes through from the starting leaf to the next older one that is immediately below the starting leaf.

•  Common types are: 1/2, 1/3, 2/5, 3/8

•  The numerator is the number of passes around the stem.
•  The denominator is the number of leaves passed through.

•  Usually, both the numerator and denominator are members of the Fibonacci series.

•  The Fibonacci series

•  0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ...
•  Each number is the sum of the previous two numbers.
•  The Fibonacci series shows up in many biological patterns where spirals are involved.
•  The Fibonacci spiral
Fibonacci sprial
The Fibonacci spiral drawn by connecting the corners of tiled squares where the side of each square is a Fibonacci number (1, 1, 2, 3, 5, 8, ...)
•  Nautilus shell is an example of a logarithmic spiral in that approximates the Fibonacci spiral
Nautilus shell
The shape of the nautilus shell approximates the Fibonacci spiral.
Photo by Chris 73/Wikimedia Commons
•  Much significance has been attributed to patterns that seem to be based on the Fibonacci series, but (at least for phyllotaxy) it just reflects efficient packing of the leaf primordia.

•  Contact parastichies

•  A more meaningful interpretation can be done by analyzing leaf primordia around the periphery of the shoot apex.

Apium shoot apex, SEM
Scanning electron micrograph of Apium graveolens shoot apical meristem.
Micrograph by Roger Meicenheimer

•  Before the stem elongates, primordia are packed together, making contact with each other.

•  Tracing spirals through young primordia to older ones that in contact with them, reveals characteristic spirals, one set running clockwise and another set counterclockwise.

•  The number of spirals in each set are usually successive numbers in the Fibonacci series.

•  Top view of shoot apex with leaf primordia

Computer generated diagram showing positions of leaf primordia around shoot apex.
Diagram by John Tiftickjian

•  Same apex with contact parastichies marked.

Computer generated diagram showing positions of leaf primordia around shoot apex with contact parastichies marked.
Diagram by John Tiftickjian

•  Another view, both sets of parastichies shown

8-13 Phyllotaxy
Computer generated diagram showing positions of leaf primordia around shoot apex with contact parastichies marked.
Diagram by John Tiftickjian

•  When many primordia are produced with little vertical growth, parastichies become more obvious, and the number of spirals increases.

•  Sunflower head with 21-34 phyllotaxy

21-34 Phyllotaxy
Sunflower head showing parastichies of florets (21-34)

•  Branching

•  Dichotomous branching

•  Branches of equal size and diverge away from each other at the same angle.

•  Common in lower vascular plants (Psilotum, Lycopodium)

•  Apical meristem divides into two equal sized segments.

•  Each segment becomes the apical meristem of a branch.

•  Axillary branching

•  Branches occur at the nodes.

•  Axillary meristems (buds) form just above each leaf primordium.

•  Each axillary bud (usually after dormant period) becomes the apical meristem of the branch.

•  Adventitious branching

•  Sometimes buds arise in positions other than in leaf axils - roots, old stems, even leaves sometimes.

•  Branches from these buds are called adventitious.

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