Processes of plant development

Question 1

What are the two types of responses to environmental changes?

Answer 1

Rapid and reversible (most animals & some plants)
Slow and irreversible (most plants)

Question 2

Where do cells in a plant trace their origin to?

Answer 2

Apical Meristem

Question 3

What do plant embryos consist of?

Answer 3

Cotyledons (seed leaves)
Shoot apical meristem (SAM)
Root Apical meristem (RAM)

Question 4

What is the name of the short ‘stem’ that separates the SAM from the RAM?

Answer 4

The Hypocotyl

Question 5

What two types of cells are generated from apical meristems?

Answer 5

Cells that continue to divide, maintaining the meristem

Cells that stop dividing and build the plant

Question 6

What do types of cells do the arresting cells fall into?

Answer 6

-Cells that promptly expand and differentiate

Cells for which arrest is a temporary phase, and occupy fixed positions, then resume cell division again later to build additional structures: branch roots, branch shoots & extra girth.

Question 7

What is the Apical meristem?

Answer 7

A small (approx 100-250um) dome of dividinf cells at the very tip of every shoot or root.

Both root and shoot apical meristems consist of a central group of cells the function of which is thought to be organisational and outer layers of cells dividing rapidly to generate new tissue.

Question 8

Root apical meristem

Answer 8

The central cells in root AM are called a quiescent centre (qc). The meristematic cells form 2 bowl shaped layers: the root initials (ri) and the cap initials (ci).

Question 9

Angiosperm shoots

Answer 9

The central zone grades off less distinctly into the meristematic regions. The zonation is easier to see in gymnosperm shoot apical meristems because the ‘central mother cells’ divide more slowly then their angiosperm counterparts.

Question 10

Leaf Morphogenesis

Answer 10

Leaf shape and size is generated by a sequence of meristems, each independently controlled.

During morphogenesis, a not yet fully formed leaf is known as a primordium

Question 11

Phase 1 of leaf morphogenesis (Determination of LF position)

Answer 11

Accelerated cell division & expansion of some few cell layers under the surface of the flanks of the apical dome which heaves up a bump or buttress.
The position of this is determined only by the 2 nearest primordia (inhibitory) so that the leaves arise in a constant arrangement with respect to older leaves.
This pattern of leaf position/arrangement is known as the phyllotaxis.

Question 12

Phase 2 of leaf morphogenesis (Determination of LF length)

Answer 12

The +/- random divisions of the first phase stop, & growth is taken over by a cell or small number of cells just under the outmost layer.

Proximal (back) cells stop dividing
Distal (front) cells keep dividing and the bump or buttress grows to form a peg which will expand to become the mid-rib.

The cells of the apical meristem of the leaf cease dividing & vacuolate before the leaf is 1mm long in dicots, 0.3, in gymnosperms and monocots.

Question 13

Phase 3 of leaf morphogenesis (The marginal meristem) (Determination of LF width)

Answer 13

A stripe of cells down each side of the peg & extending 4 cells into it re-commence active cell division.
The accumulated cells form 2 wings laterally, with cell division concentrated along the margin of the growing, curved plate which will later expand to become the leaf blade or lamina.

Question 14

Phase 4 of leaf morphogenesis (Plate enlargement) (Determination of LF width)

Answer 14

The plate is enlarged by a phase of anticlinical divisions scattered throughout.

Anticlinical divisions add to the area
Perclinical divisions add to the thickness

Question 15

Phase 5 of leaf morphogenesis (Determination of LF thickness)

Answer 15

A sudden wave of periclinical divisions from the tip to the base generate the final 7 to 10 cell thickness.

Question 16

Different leaf shapes

Answer 16

-Different leaf shapes are achieved by restricting the duration or distribution of individual phases (conifer needles are generated by a similar sequence except the activity of the marginal meristem is very limited.

-Differences between spp are genetically determined, but phases 4&5 can be under environmental control for production of specific features.

Question 17

Expansion & Differentiation in leaves

Answer 17

The cell division phase is followed by a phase of cell expansion & differentiation in which the spongy mesophyll cells stop expanding earlier than other cells and therefore get ripped apart and give arise to large intercellular spaces.

Cell expansion stops first in the LF tip, passing as a wave back down to the petiole, if there is one, whose entire development is very much later than the rest of the leaf.

The cell expansion phase of the petiole often lasts the entire life of the LF so that growth adjustments for the direction of light can be made.

Petioles are supported by collenchyma, whose walls thicken very early, & phloem fibres (raffia palm)

Question 18

Cell Expansion (Cell walls)

Answer 18

The cell walls in tissues which are still expanding are known as primary walls.
The primary walls are thin and consist of cellulose microfibrils in a pectin & water matrix (gel).
When cell expansion is finished, the thicker secondary wall is laid down over the inside of the wall in certain cell types, except where there are clusters of plasmodesmata - pit fields. These ‘windows’ in the secondary wall are known as simple pits.

Question 19

Cell expansion (Secondary walls)

Answer 19

Secondary walls are made mostly of cellulose microfibrils with a much smaller proportion of matrix polysaccharide and water and contain lignin.
This type of cell wall provides strength, but puts a stop to cell expansion.
Hence there are problems in providing mechanical support for the growing primary tissues.

Question 20

Fibres

Answer 20

One of the ways to provide mechanical support for growing primary tissues.

Fibre cells belong to the class of support tissue known as sclerenchyma with massively thickened hard, cellulose walls, often but not always lignified.
The wall is inextensible so the cell goes on extending to cope with the expansion of the surrounding tissue by apical intrusive growth.

The ends of the cell are pointed, thin walled and cytoplasmic. They carry on growing, boring intrusively between the expanding cells while the middle section thickens up.

Question 21

Collenchyma

Answer 21

Another way to provide mechanical support for growing primary tissues

These cells have very thick but extendable walls. Beginning at the corners often, parenchyma cells lay down a wall of alternating layers of pectin & cellulose which is strongly capable of supporting up to 10-12kg/mm(2) before breaking.

Collenchyma is distributed in stems as a strengthening tube. The cells are very long and thin in LS by the time stem elongation is complete.

Question 22

Size-determining meristems in the stem

Answer 22

Stems have various subsidiary ‘meristems’-: zones of cell division basal to the shape determining apical meristem.

One of these, known as dicots as the subapical meristem, controls internode elongation in a simple, quantitative way.
Divisions are predominantly anticlinical (add to the length) but not to the thickness of the axis.
Seperating out this activity keeps the upper internodes very short and as a result the developing leafs protect the apical dome and younger leafs.