Plant growth and development

IX. Plant Growth and Development

A.Growth from Seeds

1. Germination
- seed contains embryo in stage of arrested development
- germination = resumption of growth; process governed by environmental factors and genes
- environmental factors: no. of hours of daylight, availablity of water, soil temperature, oxygen levels; example: imbibing water needed to cause seed coat to split --> more oxygen reaches embryo to allow aerobic metabolism
- root cells activated first (usually) --> cell division and elongation --> primary root breaks through seed coat (= end of germination
2. Primary growth patterns
- cell division increases capacity for growth, but only about 1/2 of daughter cells undergo enlargement [Text fig. p. 532]
- water uptake drives cell enlargement; when plant cell enlarges, cell wall thickened by polysaccharide deposits
- selective expression of genes results in differentiation of daughter cells and determines specifics of subsequent growth; descendent cells divide in prsecribed planes and expand in specific directions --> plant parts of specific shapes, functions (e.g., leaf formation)
- plant hormones produced when some cells express genes for those hormones
3. Early development of corn, Phaseolus [Figs. 32.2b, 32.3]

B.Growth in "adult" plants

1. Apical meristems --> primary growth (shoot and root)

2. Lateral root formation [Fig 29.19]
- lateral root develops from pericycle; cells divide, form a clump which elongates and pushes out through the root cortex
- vascular cylinder of lateral root retains connection with the "stele" (central vascular cylinder) of primary root
[This occurs in taproot system of dicots. Why is this not a good discription of what happens in monocots? (No taproot.)]
3. Secondary growth (woody plants)
- occurs during second and successive growing seasons in all gymnosperms, most dicots; rare in monocots
- growth originates in vascular cambium, cork cambium (= lateral meristems) [Fig. 29.20]

a. vascular cambium --> secondary xylem, phloem
- vascular cambium = parenchyma cells that retain ability to divide; one, or a few cells thick; each growing season v.c. cells divide (mitosis) --> one daughter cell differentiates, the other remains meristematic [Fig. 29.21]
- xylem forms on inner face of v.c.; phloem forms on outer face; early (spring) xylem cells have relatively large diameters and thin walls compared to xylem produced later in the summer (=late wood); this alternation of larger, thin-walled cells and narrower, thick-walled cells = annual growth layers (tree rings) [Fig. 29.24]
- wood = tracheids, vessel elements and fibers
b. cork cambium --> cork
- during secondary growth, epidermis from primary growth splits, dries and falls off stem
- cells in outer cortex (outside secondary phloem) become meristematic and divide to form cork cells on outer perimeter; cork cells deposit waxy material (= suberin) in their cell walls, then die; cork = layers of dead cells
- preiderm = cork cambium + cork; replaces epidermis, provides protection against pathogens, waxy walls retard water loss (bark includes periderm and phloem)

C.Plant Hormones [Table 32.1]

- as in animals, plant hormones = chemicals for communication;
- hormone = a chemical released from one cell that affects growth, development of target cells which have appropriate receptors
- unlike animal, hormones that act on distant cells, plant hormones can act on adjacent cells as well as distant ones
- there are five known catagories of plant hormones:

1. Auxins
- produced by shoot apical meristem tissue
- promotes cell elongation in stems, coleoptiles (= protective sheath covering monocot shoot during germination) [Fig. 34.4]
- promotes development of lateral roots, even at very low concentrations; may also participate in stem, root growth responses to light, gravity
- inhibits lateral bud sprouting = apical dominance
- IAA (indole acetic acid) = impt. natually occuring auxin; applied to fruit trees to promote flowering, set fruit and encourage synchronous development of fruit
- synthetic auxins can be used as herbicides (see Focus essay, p. 535)
2. Gibberelins
- also promote cell elongation; prod buds and seeds into breaking dormancy and resuming growth in spring; stimulate breakdown of starch (may influence flowering in some species)
3. Cytokinins
- stimulate cell division in root meristems where they are abundant
- correct balance w/ auxin promotes sprouting of buds
- promote leaf expansion; retard leaf aging (used commecially to prolong life of stored vegetables and cut flowers)
4. Abscisic acid
- inibits cell growth; helps prevent water loss by promoting stomatal closure; promotes seed, bud dormancy
- applied to nursery stock prior to shipping to maintain dormancy and thus resist damage
5. Ethylene
- stimulates ripening of fruit (used commecially for this purpose)
- promotes "abscission" of leaves, fruit, flowers causing them to drop from plants at appropriate times of year