Lecture 17 

Stems - Primary Structure

I. Introduction

A. Shoot = stem + leaf.  System is more complex than roots.

B. Stems are extremely varied in plants, with many modifications. Aspects that affect this variation include:
1. leaf arrangement
2. leaf insertion
3. presence / absence of axillary buds
4. where (at what level) does branching occur
5. is the shoot vertical or horizontal, free-standing or climbing, etc.
6. where does the shoot system grow? (aerial, underground, in water)

II. Primary structures

A.  Three tissue systems: dermal, ground (fundamental), vascular (fascicular)
1.  Epidermis – covered in Lecture 7
2.  Cortex.  Often with chlorenchyma.  Aerenchyma in aquatics. Peripheral parts with collenchyma (dicots) or sclerenchyma (grasses).  Figure 16.2. Some angiosperms and lower vascular plants form a Casparian strip on a stem endodermis.
3.  Pith.  Parenchyma cells with or without chloroplasts.  Often destroyed with continued growth of the stem.  Nodes may retain the pith as a diaphragm, also internodes (see winter botany photos).  Idioblasts may occur in the pith.
4.  Vascular system in dicots and gymnosperms it is a hollow cylinder (see stele types below).  Inside is pith, outside cortex  Vascular bundles separated by ground parenchyma – interfascicular. This connect the pith and cortex (Figure 16.1) = medulary or pith rays. Monocots (Figure 16.2) – vascular bundles don’t form a ring, hence no cortex and pith.

B.  Vascular system.  Discrete individual strands = vascular bundle.  Can be:
1. colateral – phloem occurs external to xylem - abaxial (Figure 16.1B, 16.2B)
2. bicolateral – phloem occurs on each side of xylem (Figure 16.9A), i.e. external (abaxial) and internal (adaxial) phloem. Occurs in Apocynaceae, Convolvulaceae, Solanaceae, Cucurbitaceae (Cucurbita image).
3. amphicribral – phloem surrounds the xylem.  Occurs in ferns (Pteridium rhizome XS) and some angiosperms (flowers).
4. amphivasal – xylem surrounds phloem.  Often in medullary bundles of Rheum, Rumex, Begonia as well as some Araceae, Liliaceae, Juncaceae (Juncus image) and Cyperaceae.

III. Leaf Arrangement and Vascular Organization

A.  Patterns of vascular strands in stem reflect relationship with leaf.  At node, one or more vascular strands leave the stem and enter the leaf.  These are called leaf traces.

B. Leaf phyllotaxy (Diagram)
1. One leaf per node. Generally called alternate, but two different types:
a. distichous (2-ranked), arranged in two rows, 180 degrees apart. Figure 16.5 of Zea stem cross section. This type has orthostichies.
b. helical; another form of alternate This type has parastichies.
2. Two or more leaves per node. These have orthostichies.
a. opposite.  Two leaves per node.  If alternating 90 degrees every node, called decussate.  If less than 90 degrees, called bijugate.
b. whorled, three or more leaves per node.

C. Fibonacci Series, the Golden Ratio and other cool stuff
1. The Fibonacci Series is the series of fractions such as 1/2, 1/3, 2/5, 3/8, 5/13 … For plant phyllotaxy, this is the # of windings around the stem over the number of leaves encountered.  Note that each successive fraction can be obtained by adding the numerators and denominators of the preceeding two fractions.

2. If one kept extending the Fibonacci series, these fractions approach (but do not reach) 137˚ 30’ 28” (Diagram).  If this angle is projected onto a circle, the sector (A) has particular properties.  The ratio of A to the remaining part (B) is the same as B compared to the entire circle (either circumference or area can be used).  For example, if A = 1, B = 1.61803…, actually, an irrational number called [phi = 1 + square root of 5 / 2].  The ratio 1 to phi is called the Golden Ratio. Wiki page.
3. Many things in nature conform to the Golden Ratio, such as the nautilus shell, and in our case, the stacking of leaves around a stem.  In the case of 5/13 phyllotaxy, leaf n and n + 13 are directly above each other. The 5/13 phyllotaxy is in Figure 16.3C. Why do leaves tend to obey the Golden Ratio? At least two explanations:
a.  The resulting phyllotaxy was selected for to avoid placing one leaf directly above another, thus avoiding shading. 
b. The Golden Ratio has a relationship with the Logarithmic Spiral, as seen with the nautilus shell.  A logarithmic spiral can be extended outwards or inwards and always has the same relative shape.  For plants, a similar phenomenon occurs at the apical meristem where leaf primordia develop but occupy the same relative proportion of the apex surface initially and later through development.  The same thing can be seen in the arrangement of flowers/fruits on a pineapple, fruits on a sunflower head, or scales on a pine cone.  These units have the same shape, regardless of size.

4.  One can recognize the units occur in spirals, some clockwise, some counterclockwise.  These “genetic spirals” are called parastichies.  One stem / leaf system, for example, can have more than one parastichy, and these spirals vary in their steepness and direction. 

5. The Hectorella (Montiaceae) example, Figure 16.3A shows the apical region as seen from above. 
a. Two contact parastichies are shown: 1, 4, 7, 10, 13 and 1, 6, 11, 16.  The numbers refer to the order that the leaves are produced at the apex.  The term plastochron refers to the intervals between leaves of a given parastichy.  So in the first example, the leaves are three plastochrons apart.  In the second example, they are 5 plastochrons apart.
b. Note that the leaf traces as well as the other vascular bundles that do not go to leaves.  The latter are called stem (or axial, cauline) bundles.  Together, the leaf traces and stem bundles make up a sympodium. 
c. For Hectorella, each leaf trace is connected to two sympodia, thus the whole system (all 13 sympodia) is interconnected.  This type of vascular system is called closed. 

6. The Abies (fir, Pinaceae) example, Figure 16.4 shows the vascular system cut open and layed out flat.  It has 13 longitudinal sympodia coinciding with 13 parastichies of leaves.  Note also that the leaves in any one vertical sympodia are separated by 13 plastochrons (e.g. leaves 7, 20, 33), but they are not connected to each other. This type of vascular system is called open. 

D. Stem and leaf, thus cauline (stem) bundles and leaf traces, have no fundamental distinction.  In general, leaf traces can be distinguished from stem bundles by being wider (Figure 16.6).  Also, have transfer cells in vascular parenchyma (associated with xylem).  Lateral transfer of nutrients takes place among cells in the apical and lateral meristems.

E. Monocot vascular anatomy.  Reconstruction of the 3-D anatomy of vasculature in monocots can be complex because they have so many vascular bundles.  Serial sections can be recorded on video and then played back to see progression of changes in the tissues.  Figure 16.7 shows a reconstruction of the palm Rhapis excelsa.

F. Leaf gap
1. In the vascular cylinder, a leaf gap is parenchyma above a point where the leaf trace diverges from the stem bundle. Diagram of a fern leaf gap. If the vascular cylinder already has lots of interfascicular, axially elongated regions, then distinguishing the leaf gap (that merges with these) is difficult and arbitrary (Figure 16.8).
2. The leaf gap / leaf trace anatomy can be categorized based on the numbers of parts. Here gap = lacuna:
a. unilacunar – one leaf trace and one gap
b. trilacunar – three leaf traces and three gaps
c. multilacunar – many leaf traces and many gaps
3. The two-trace unilacunar type has two traces for one lacuna, but the traces derive from different sympodia

G. Branch traces and branch gaps. Branch traces shown in Figure 16.8D.  The branch trace is the vascular bundle connecting the stem with the branch. These are actually leaf traces that go to the prophylls – the first leaves that flank a lateral branch.

IV. Stele Types

A. In roots the stele is represented by everything inside and including the pericycle. In stems the stele is all of the vascular system (interior to the cortex), i.e. primary tissues - xylem, phloem, pericycle (derived from the procambium). Whether the endodermis is included or not varies.  Stele types are distinguished by the arrangement of the vascular tissue in cross section of the root or stem (including modifications such as rhizomes). 

B.  Types of stelar anatomy (protosteles and siphonosteles)
1. Protostele. A solid cylinder of vascular tissue in center, found in lycophytes, ferns, stems of water plants, roots of seed plants.

a. Haplostele - in cross section the xylem is circular and surrounded by phloem in a ring. Image of medulated (with parenchyma intermixed) haplostele of Gleichenia rhizome.

b. Actinostele - in cross section the xylem is star shaped and there are from 3-6 or many points (in monocots) to the star, the points are the areas of protoxylem.  We saw several variants of actinosteles in the lecture on root primary structure [recall Fig. 14.9 diarch, triarch, tetrach, pentarch, and polyarch from Primary Roots lecture].

c. Plectostele - xylem scattered but in groups, surrounded by phloem as in lycophytes. Image of Lycopodium.

2. Siphonostele - with pith or ground tissue internal to vascular tissue. Imagine it as a cylinder with parenchyma in the center (pith).

a. Ectophloic siphonostele – the phloem is on the outside of the vascular cylinder, e.g. Botrychium (grapefern) and several dicots. Image of a dissected ectophloic siphonostele in an Osmunda (fern) rhizome.

b. Amphiphloic siphonostele – the phloem is on both sides of the vascular cylinder. Image of Marsilea (a fern). Two main types:

1) Solenostele - an amphiphloic siphonostele without overlapping leaf gaps. In cross section at an internode it has a complete cylinder of vascular tissue, itself surrounded by phloem, surrounding a pith. Examples: Adiantum, Dennstaedtia.
2) Dictyostele – a dissected amphiphloic siphonostele. It is dissected because there are many leaf gaps in every cross section. Many anastomosing strands each surrounded by phloem. Image of Pteridium rhizome XS.

c. Eustele – “true stele”. Image of Medicago. Some features include:
d. Atactostele – Bundles of vascular tissue that are scattered in the ground tissues, e.g. monocots such as Zea.

C.  Possible evolution protostele to eustele for progymnosperms and a gymnosperm (Figure 16.10). Series ends with a conifer with leaf-gap like anatomy (openings in the anastomosing strands).

Last updated: 14-Oct-22 / dln