Dr. T logo BIO 410/510 Plant Anatomy
The stem


Review typical dicot stem cross section for tissue regions

•  Epidermis

•  Cortex

•  Parenchyma

•  Often chlorenchyma

•  Collenchyma

•  Present in most stems just under epidermis

•  Sclerenchyma

•  Present in some plants

•  May be ring of perivascular fibers that surround vascular bundles.

•  Vascular bundles in single ring

•  Xylem on inside facing pith

•  Phloem on outside facing cortex

•  Tissue between bundles is called interfascicular tissue or a medullary ray

•  Pith

•  In most plants contains only parenchyma

•  Typical dicot stem, cross section, low magnification

Helianthus stem, x.s.
A sunflower stem with its vascular bundles in a single ring, a feature typical of dicots
Micrograph by John Tiftickjian

•  Typical dicot stem, cross section, higher magnification

Helianthus stem, x.s.
Cross section of Helianthus (sunflower) stem with major tissues labeled
Micrograph by Biodisc

Vascular bundle types

•  Collateral

•  Xylem and phloem aligned along a radius; xylem inside, phloem outside

•  Bicollateral

•  Xylem and phloem aligned along a radius; phloem outside, xylem in middle, phloem inside

•  Concentric

•  One vascular tissue is surrounded by the other

•  Rare in seed plants; more common in lower vascular plants

•  Amphicribral bundle. Phloem surrounds xylem.

•  Amphivasal bundle. Xylem surrounds phloem.

Leaf traces and leaf gaps (nodal anatomy)

•  3-dimensional structure of vascular system at a node

•  Nodal anatomy reflects how the vascular tissue of the stem interconnects with the vascular tissue of the leaf.

•  A leaf trace is one (or several) vascular bundles that lead to a leaf.

•  The leaf traces may branch repeatedly to form the veins of the leaf.

•  Leaf traces may or may not be associated with leaf gaps.

•  There are two basic leaf types in vascular plants: microphylls and megaphylls

•  Microphyll

•  Leaf is relatively small ("micro" + "phyll")

•  A leaf has only one vein (leaf trace does not branch)

•  There is no leaf gap.

•  Found in one division of plants - the Lycophyta (sometimes called the microphyllophyta)

•  Megaphyll

•  Leaf is relatively large ("mega" + "phyll").

•  A leaf has a system of branching veins.

•  There is a leaf gap present at the node opposite the leaf trace.

•  Found in all division of plants with leaves except the Lycophyta. (Ferns, seed plants)

•  Nodal anatomy of microphylls vs. megaphylls

Microphylls and megaphylls
Comparison of microphylls and megaphylls.
Modified from illustration in Brooker Biology textbook, Copyright McGraw-Hill companies

•  For microphylls, the leaf trace branches directly off from the stem's vascular system. There is no break in the vascular cylinder of the stem.

•  For megaphylls, there is a "gap" in the vascular cylinder of the stem immediately above each leaf trace at the node. The gap is filled with ground tissue, usually parenchyma.

•  3-D models demonstrated in lecture.

•  How does the 3-dimensional geometry of the leaf gap (or lack of leaf gap) affect the cross sectional view of the node?

•  Cross sections of stem without leaf gap (microphyll)

Microphyll has no leaf gap
Cross section stem at both a node and an internode for plant with microphylls (no leaf gap)
Drawing by John Tiftickjian

•  Cross sections of stem with a leaf gap and stem vascular cylinder in complete ring.

Megaphyll has leaf gap
Cross section of node and internode for plant with megaphylls (leaf gap present). The stem vascular tissue is a complete cylinder.
Drawing by John Tiftickjian

•  Cross sections of stem with a leaf gap stem vascular cylinder of separate bundles.

Separate bundles and leaf gap
Cross section of node and internode for plant with megaphylls (leaf gap present). The stem vascular tissue is dissected into separate vascular bundles.
Drawing by John Tiftickjian

•  Microphylls and megaphylls evolved differently.

•  Microphylls evolved as simple outgrowths of the stem (called enations or prophylls).

•  Once they developed a vascular trace, they had become true leaves.

•  Megaphylls evolved from systems of branches.

Megaphyll evolution
Steps in the evolution of megaphylls
Modified from illustration in Brooker Biology textbook, Copyright McGraw-Hill companies

•  Ancestral leafless plants had dichotomous branching

•  The branching pattern changed so that branches were unequal (one branch "overtopped" the other). This led to the stem having a main axis with smaller side branches.

•  The next step was restriction of the side branches to one plane (planation).

•  Growth of tissue between branches led to the leaf blade (webbing).

•  The veins of the leaf had thus evolved from stem branches.

•  It is thought that the leaf gap reflects this evolutionary pathway.

The stele concept

•  The stele is defined as all tissues inside the endodermis (when present).

•  The endodermis

•  The endodermis is a distinct layer of cells, one cell thick, separating the cortex from the vascular tissues.

•  By definition, the endodermis is the innermost layer of the cortex.

•  The endodermis plays a roll in regulating the movement of materials from the cortex to the vascular cylinder.

•  Although an endodermis is common in the stems of seedless vascular plants, it is usually absent in stems of seed plants.

•  Roots of seed plants do have an endodermis.

•  Casparian strips

•  A distinguishing feature of endodermal cells is the presence of special structures within their cell walls known as Casparian strips.

•  Casparian strips allow the endodermis to regulate transport between the cortex and the stele?

•  We will cover Casparian strips in more detail later in the section on roots.

•  Stele types

•  Protostele

•  No pith present

•  Common in Psilotum, Lycophyta, some ferns

•  Haplostele. Vascular tissue has a circular outline-xylem in center surrounded by phloem.

•  Actinostele. Similar to haplostele, but xylem has star-shaped outline in cross section.

•  Plectostele. Several plate-like areas of xylem that alternate with areas of phloem.

•  Siphonstele

•  Pith (ground tissue) present in center of stele

•  Vascular tissue in continuous ring (not "dissected")

•  Common in ferns
•  Amphiphloic siphonostele. Phloem both inside and outside of xylem
•  Ectophloic siphonstele. Phloem only outside xylem

•  Vascular tissue in bundles ("dissected")

•  Bundles are present because leaf gaps overlap vertically.
•  Dictyostele
•  Vascular system is a ring amphicribral bundles (also called meristeles)
•  Very common in ferns
•  Eustele
•  Vascular system is a ring collateral bundles (sometimes bicollateral bundles)
•  The typical stele type nearly all dicots and gymnosperms.
•  Atactostele
•  Vascular bundles not in a single ring but scattered throughout the stem.
•  The standard stele type in monocots.

•  Identification guide for stele types

Stele types
Comparison of stele types

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