Plant Growth and Development

Meristems and leaf development

Meristems in Plant Development

•Small isodiametric cells with embryonic characteristics•Retain their embryonic character indefinitely•Some differentiate while others retain capacity for cell division

Stem cells: cells that retain their capacity for cell division indefinitely

Shoot Apical Meristem

Stem Leaves and lateral buds

Shoot apex: apical meristem+leaf primordia

•Shoot apical meristem can contain a few hundred to a thousand cells but Arabidopsis SAM has about 60 cells•Small thin-walled cells, dense cytoplasm, lacks large central vacuole•Grows rapidly in spring-slow growth during summer-dormant in winter

Shoot Apical Meristem Structure

internal tissues of

stem

Like Quiescent center in

roots

Cytohistological Zonation

Postembryonic Meristems

Primary meristems1. Root meristem 2. Shoot meristem

Secondary meristems1. Axillary2. Inflorescence 3. Floral4. Intercalary5. lateral

Preembryonic Meristems

Axillary •Formed in the leaf axils•Derivative of shoot apical merstem•Produce branchesIntercalary•Found within organs, near their bases•Enables grasses to continue to grow despite mowing or grazing

Branch root•Formed from pericycle cells in mature root regionsCork Cambium (Lateral meristem)•Develops within mature cortex cells and secondary phloem•Periderm or Bark are its derivative layers that form outer protective surface in woody trees

Vascular Cambium (Lateral meristem)Woody tissues of stems and roots

Fusiform Stem CellsHighly elongated, vacuolate cells that differentiate into the conducting cells of xylem and phloem

Ray Stem CellsSmall cells whose derivatives include the radially oriented files of parenchyma cells within wood known as Rays

Floral meristems•Produce floral organs such as sepals, petals, stamens and carpals•DeterminateInflorescence meristem•Produces bracts and floral meristems in the axils of bracts•Could be determinate or indeterminate

Determinate meristems:Genetically programmed limit to their growthIndeterminate:No predetermined limit to growth

Bracts:A leaf from the axils of which a flower or floral axil arise

Consists of one or more leaves, the node to which leaves are attached, internode and one or more axillary buds

Could also be apical meristems provided they get the developmental potential

Leaf Development

Axil Development• leaves are lateral organs.• leaves display consistent orientation and

polarity relative to the shoot i.e. axial information in the leaf does not arise de novo but depends on existing axial information.

• Angiosperm leaf is almost always a determinate organ.

Stages of leaf development

1- Organogenesis:•Leaf founder cells formed by L1 and L2 layers of apical meristem, produce leaf primordium that ultimately develops into leaves2- Development of suborgan domains•Primordium differentiates into specific leaf parts

• Dorsiventral (abaxial-adaxial)• Proximodistal (apical-basal)• Lateral (margin-blade-midrib)

3- cell and tissue differentiation•L1 layer forms epidermis•L2 layer forms photosynthetic mesophyll cells•L3 layers forms vascular elements and bundle sheath cells

Simple leaves have three axes of symmetry.• proximodistal axis from base of the leaf to the tip.• adaxial-abaxial axis from the upper to the lower

epidermis.• centrolateral axis from the midrib to the margin.

Structural symmetry in the leaf

Leaf Primordia Arrangement

Phyllotaxy: The arrangement of leaves around the stem

Single leaf Paired leaf Opposite leaves per node at right angle to each other

More than two leaves per node

Spiral arrangement of leaves