Lecture 18

Stems – Meristems and Development

I. Shoot Apex

A. Can be vegetative or reproductive.  We are focusing now on the vegetative growth of the stem apex (reproductive later)

B.  Shoot apex produces lateral organs = leaves.  First organized when the plant is still an embryo in the seed.

C.  Developmental order – initials and derivatives.  Organogenesis (formation of organs, like leaves) and histogenesis (differentiation of tissues, like xylem and phloem).  Both processes occurring, marked by different stages at increasing distances from the apex.

D.  Components of primary meristems:
1.  protoderm → epidermal system
2.  procambium → vascular system
3.  ground meristem → fundamental system

E.  Apical Meristems with apical cells
1. A single terminal cell called the apical cell that gives rise to all cells. Often lenticular or pyramidal in shape. All cell lineages (cell division lines) can be traced back to this one cell.
2. Present in ferns
3. What is the evolutionary selective advantage to having an apical cell? It divides less rapidly than its derivatives, thus helping prevent the multiplication of abnormalities.
4. Tetrahedral apical cell (with 4 sides) found in bryophytes ferns and lycophytes (Takakia). Typically 137˚ between the midlines of apical cell segments, therefore the cells will be coming off at 137˚ around the stem and this results in a spiral arrangement.
5. Row of initials - found in some bryophytes. These more complicated apical meristems have numerous cells at the apex that produce the primary meristems.

F.  Tunica-corpus organization 
1. Seen in angiosperms and some gymnosperms (Araucariaceae, Ephedraceae). Most gymnosperms – outer layer divides anticlinally and periclinally, contributing cells to periphery and interior (3c below).
2. A shoot apex of a typical angiosperm is Solanum tuberosum (Figure 16.11) at two stages. Different regions show cells that differ in size, vacuolization, orientation of mitoses. These cells further differentiate farther downward and organogenesis takes place.
3. Features of the tunica-corpus
a.  Separate tiers of apical initials occur at the distal position at the apex = stratified meristems.  Outermost layer, the tunica is one or more layers thick and it divides anticlinally. 
  • In Zea (maize) the tunica is one-layered (Figure 16.15, image)
  • In Elodea the tunica is also one-layered (image)
  • In Coleus the tunica is two-layered (Figure 16.13). Image. Image from Mauseth web page, with interpretation. Animation from Meicenheimer web page.
  • In Salvia (sage) the tunica is multi-layered. Image.
b.  Corpus is several cell layers deep, divides in various planes (Figure 16.14). Corpus provides the bulk, tunica the mantle or covering of the shoot. Prominent corpus shown in this image of Aesculus.
c. Esau equates tunica-corpus with core and mantle, but Mauseth distinguishes them (latter for gymnosperms) because the outer layer undergoes some periclinal divisions.
6.  Support for the concept that the two layers of the tunica and the corpus are independent comes from work with periclinal chimeras (produce polyploidy nuclei, thus can trace cell lineages) as well as cells with mutant chloroplasts. Destinies of the cells are not predetermined at apical meristem, even if tunica-corpus are independent.  Epidermis – yes, usually formed from outermost layer, but deeper layers are not fixed.  This contrasts with the histogen theory of Hanstein (proposed dermatogen, periblem and plerome).

G.  Cyto-histological zonation
1. Refers to the differentiation of regions with distinctive cytological characteristics. First studied in gymnosperms such as Ginkgo, it also applies to angiosperms. Esau uses the term shoot apex which includes both the apical meristem and derivative meristematic regions.
2. Pinus (Figure 16.16A and Figure 16.17). Images from Mauseth web page. Shoot apex consists of:

H.  Identity of of apical initials
1. Roots have quiescent centers, but what about shoots?  French cytologists say “yes”.
2. The distal axial cells are inert, the actual active zones are peripheral and subterminal where stem tissue and leaf primordia arise. 
3. Counts of mitosis (Figure 16.18) confirm this idea (distal zones less active). 
4. Whether cells function as initials or not depends upon their position (not inherent properties).
5.  Newman – no cells are permanent initials.  Classification of apical meristems:
a. monopodial (ferns).  Figure 16.12.  Residue in superficial layer and any kind of division gives increased length and width
b.  simplex (gymnosperms).  Figure 16.17. Residue in superficial layer and both anticlinal and periclinal divisions give the bulk of growth.
c. duplex (angiosperms).  Figure 16.14.  Residue in at least two surface layers with two contrasting modes of growth: anticlinal near surface, at least two planes in deeper layers.

II.  Origin of Leaves

A. Protrusion on shoot apex = leaf buttress. Arrangement (position) based on phyllotaxy.  Decussate shown in Figure 16.19 for Kalanchoe.  Peripheral region most active. Periclinal divisions in second and third layers initiates leaf primordia (Figure 16.11) and anticlinal divisions on surface keep pace with with dividing cells underneath.

B.  Tunica alone or both tunica and corpus can be involved in leaf formation. For Solenostemon (Coleus) (Figure 16.13), both tunica and corpus contribute.

C. What determines where a new leaf is initiated? First Available Space Hypothesis. This concept states that physiological and physical constraints at the apex determines what space is available for the next leaf.  But leaf position is part of an overall pattern of shoot organization [that is genetically determined] that involves vasculature (stem and leaf trace) architecture. These are factors outside the apical regions that influence leaf position.

III.  Origin of Branches

A.  Axillary buds.
1.  Axillary buds often form ca. 2-3 nodes down from the apex. For some buds, the active tissue is in continuity with the peripheral zone of the shoot apex and is therefore called an attached type. A good example of this is the image of Oroya cactus. If buds are initiated in later plastochrons, they are called detached.
2.  Basal and lateral portions of the the incipient buds divide actively, called the shell zone (Figure 16.20). 
3.  First leaves formed by the axillary bud meristem are called prophylls (Figure 16.9A).
4.  Adventitious buds can form almost anywhere, exogenously (externally, e.g. from epidermis) or endogenously (internally). If forming from mature tissues, requires cells to de-differentiate.
5. Axillary buds under the influence of the terminal shoot may remain dormant (via apical dominance) or be “released” if that terminal shoot is removed, thus forming lateral branches.

B.  Primary growth of the stem
1.  Initially, nodes and internodes not well-differentiated.  Expansion between the nodes gives rise to internodes. Good example is a grass such as Agropyron (Figure 16.22).  File-meristem growth in pith and cortex – repeated transverse divisions; later cell expansion.
2. Rosette plants – growth form is because internodes don’t expand. 
3. In monocots and some dicots, intercalary meristems  occur – between tissues that are more advanced in development.  In grasses that have lodged (fallen over), cells in the pulvinus allow the stem to be lifted off ground.  This results in stretching of cells, destroys mature tracheary elements which are replaced with newly differentiated ones or lacunar spaces.  Same for sieve elements.
4. Primary thickening. Growth is concentrated in narrow regions along the periphery of the stem – called primary thickening meristems (have orderly, anticlinal series of cells).  Occurs also in some rosette forming dicots and succulents such as Opuntia cacti.  Figure 16.15 of Zea (not very clear).
5.  Growth in large monocots
a. Cordyline (Laxmanniaceae, photos). Figure 16.23.  Primary growth alone would result in an obconic stem – unstable.  Secondary growth compensates by widening base.
b. Palms (Arecaceae). Look at the width of the base of this palm (Butia) and the width higher up. How do you explain the difference? Figure 16.24A. Remember, palm "trees" have no secondary growth. Compare this cross section of the trunk of a Washingtonia palm to a dicot tree.  As a young palm develops, its underground stem first widens (this takes time), forming short internodes. This progresses until a major diameter is reached.  Later, height growth occurs (rapidly). Some palms like this Jubaea form adventitious roots at the base. The adventitious roots provide support at the base where the stem remains small in diameter, as shown in this Phoenix palm (image).
c. Pandanus (Pandanaceae).  Figure 16.24B.  Here primary growth does produce a stem wider above than below, but the plant compensates by forming prop roots (adventitious roots) to stabilize the stem.

IV.  Vascular Differentiation

A.  Figure 16.25 is a contracted version of Figure 16.3C of Hectorella, showing how the vascular system at the apex is a miniature version of the adult form.

B. Origin of procambium (image).  Figure 16.26. Steps:
1.  At first, no vascular tissue differentiation
2.  Cortex and pith begin forming, leaving a ring of residual meristem and leaf gaps. The procambium is also just now forming.
3.  More procambium strands form as does the interfascicular region.
4. Procambium differentiates acropetally – toward the tip.

C.  Origin of xylem and phloem
1.  Colateral vascular bundles (typical of eudicots).  Phloem on the outside, xylem on the inside (relative to the stem diameter). 
2.  New phloem – first appears closer to the middle of the stem - forming in a centripetal direction. Remember that as phloem differentiates, metaphloem forms towards the inside, pushing the protoxylem to the outside.
3. New xylem – first appears closer to the outside of the stem, forming in a centrifugal direction. As xylem differentiates, the metaxylem first forms to the outside, pushing the protoxylem to the inside of the stem. This is endarch development.  Note that this is opposite of what we saw in roots where metaxylem is on the inside and protoxylem to the outside – exarch development.
 4.  Figure 16.27. Phloem appears first in the leaf, before xylem.  Development of xylem progresses acropetally to the leaf base, then basipetally to connect with the stem bundles.
5. Animation of the process from the Meicenheimer webpage at MU.
 6. For bicolateral vascular bundles (e.g. Cucurbita - image) the outer phloem forms first, then the inner phloem.
Last updated: 14-Oct-22 / dln