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 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 apex: apical meristem+leafprimordia
Shoot Apical Meristem Structure CytohistologicalZonation Like Quiescent center in roots internal tissues of stem
Preembryonic Meristems Postembryonic Meristems Primary meristems Root meristem Shoot meristem Secondary meristems Axillary Inflorescence Floral Intercalary lateral • Branch root • Formed from pericycle cells in mature root regions • Cork 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 • Axillary • Formed in the leaf axils • Derivative of shoot apical merstem • Produce branches • Intercalary • Found within organs, near their bases • Enables grasses to continue to grow despite mowing or grazing
Fusiform Stem Cells Highly elongated, vacuolate cells that differentiate into the conducting cells of xylem and phloem Vascular Cambium (Lateral meristem) Woody tissues of stems and roots Ray Stem Cells Small 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 • Determinate • Inflorescence meristem • Produces bracts and floral meristems in the axils of bracts • Could be determinate or indeterminate Determinate meristems: Genetically programmed limit to their growth Indeterminate: No predetermined limit to growth Consists of one or more leaves, the node to which leaves are attached, internode and one or more axillary buds Bracts: A leaf from the axils of which a flower or floral axil arise 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 leaves • 2- 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
Structural symmetry in the leaf • 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.
Leaf Primordia Arrangement Phyllotaxy: The arrangement of leaves around the stem Opposite leaves per node at right angle to each other Single leaf Paired leaf More than two leaves per node Spiral arrangement of leaves