BSC 314 Lecture Notes - Lecture 11: Pith, Primordium, Monocotyledon
26 Jun 2018
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Primary Growth of Stems
Stems, like roots, grow in length by division and elongation of cells at their tips. The
youngest cells of stems (but not roots) are organized into two zones: the tunica and
the corpus. In the tunica, cell divisions are perpendicular to the stem axis and give rise
to a sheet of tissue several layers thick that covers the outside of the tip. Cell divisions
in the corpus are in all directions and produce an interior mass of cells. Derivatives of
cells in both tunica and corpus continue to divide and produce three
recognizable primary (transitional) meristems—protoderm, ground meristem, and
procambium—which, as they elongate and differentiate, create the three primary
tissue systems— dermal, ground (fundamental), and vascular.
Cell divisions of the apical meristem give rise to leaf primordia close to the tip and so
consistently, one after the other, that nodes and internodes can't be distinguished until
elongation and differentiation start. At the base of the leaf primordia in the internodal
region in monocot shoots, a zone of meristematic cells (an intercalary meristem)
remains undifferentiated and retains the ability to divide throughout the life of the plant,
causing elongation of the monocot leaves from the base upward.
Steles
The central cylinder of a primary plant body is called the stele. It consists of the primary
xylem and phloem tissues together with any pith that may be present. Three types
occur: 1.) protostele, the simplest, is a solid vascular core and is found in primitive
vascular plants and the roots of eudicots (but not monocots); 2.) siphonostele, a hollow
cylinder of vascular tissue surrounding a central core of pith is common in ferns;
3.) eustele, is a system of separate vascular bundles surrounding a pith and is the type
in almost all seed plants.
Types of primary bodies
Herbaceous plants—in contrast to trees and shrubs—are composed essentially of
primary tissues. If secondary growth does occur, the tissues are used for rigidity and not
conduction. Two basic variations in the primary body of eudicots include a hollow
cylinder of xylem, cambium, and phloem surrounding a central pith and, in others, a
system of discrete vascular bundles, also with xylem, cambium, and phloem, arranged
in a regular pattern between the pith and the cortex (see Figure ). The primary body of
monocots consists of vascular bundles, with no cambium, scattered in an
undifferentiated parenchyma called ground tissue.
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Document Summary
Stems, like roots, grow in length by division and elongation of cells at their tips. The youngest cells of stems (but not roots) are organized into two zones: the tunica and the corpus. In the tunica, cell divisions are perpendicular to the stem axis and give rise to a sheet of tissue several layers thick that covers the outside of the tip. Cell divisions in the corpus are in all directions and produce an interior mass of cells. Derivatives of cells in both tunica and corpus continue to divide and produce three recognizable primary (transitional) meristems protoderm, ground meristem, and procambium which, as they elongate and differentiate, create the three primary tissue systems dermal, ground (fundamental), and vascular. Cell divisions of the apical meristem give rise to leaf primordia close to the tip and so consistently, one after the other, that nodes and internodes can"t be distinguished until elongation and differentiation start.
