Chapter 35: Plant Structure and How (in general) Plants Grow
1. Plants (more so
than animals because they are rooted in place and can not move out of an
unfavorable environment) are very plastic in their responses to environmental
change and this platisticity is encoded in their
genes.
A plants structure reflects interactions with the environment at two time scales.
Over the long term, entire plant species have, by natural selection, accumulated morphological adaptations that enhance survival and reproductive success.
For example, some desert plants have so reduced their leaves that the stem is actually the primary photosynthetic organ.
This is a morphological adaptation that reduces water loss.
Over the short term, individual plants, even more than individual animals, exhibit structural responses to their specific environments.
The architecture of a plant is a dynamic process, continuously shaped by plants genetically directed growth pattern along with fine-tuning to the environment.
Even faster than a plants structural responses to environmental changes are its physiological (functional) adjustments.
Most plants are rarely exposed to severe drought and rely mainly on physiological adaptations to cope with drought stress.
In the most common response, the plant produces a hormone that cause the stomata, the pores in the leaves through which most of the water is lost, to close.
Plants have tremendous developmental plasticity.
Its form, including height, branching patterns, and reproductive output, is greatly influenced by environmental factors.
A broad range of morphologies can result from the same genotype as the plant undergoes three developmental processes: growth, morphogenesis, and differentiation.
The development of body form and organization, including recognizable tissues and organs is called morphogenesis.
The specialization of cells with the same set of genetic instructions to produce a diversity of cell types is called differentiation.
Cell expansion in animal cells is quite different from cell expansion in plant cells.
Animal cells grow by synthesizing a protein-rich cytoplasm, a metabolically expensive process.
While growing plant cells add some organic material to their cytoplasm, the addition of water, primarily to the large central vacuole, accounts for 90% of a plant cells expansion.
This enables plants to grow economically and rapidly.
Rapid expansion of shoots and roots increases the exposure to light and soil, an important evolutionary adaptation to the immobile lifestyle of plants.
The diverse cell types of a plant, including guard cells, sieve-tube members, and xylem vessel elements, all descend from a common cell, the zygote, and share the same DNA.
In spite of differentiation, the cloning of whole plants from somatic cells supports the conclusion that the genome of a differentiated cell remains intact and can dedifferentiate to give rise to the diverse cell types of a plant.
Cellular differentiation depends, to a large extent, on the control of gene expression.
Cells with the same genomes follow different developmental pathways because they selectively express certain genes at specific times during differentiation.
The juvenile-to-mature phase transition in plants highlights a difference in the development of plants versus animals.
In an animal, this transition occurs at the level of the entire organism - as when a larvae develops into an adult animal.
In plants, phase changes during the history of apical meristems can result in juvenile and mature regions coexisting along the axis of each shoot.
2. Plants
are made up of three basic organ types: roots, stems, and leaves. You need to know the general structure and
function of each organ type.
The plant body is a hierarchy of structural levels, with emergent properties arising from the ordered arrangement and interactions of component parts.
The plant body consists of organs that are composed of different tissues, and these tissues are teams of different cell types.
3. Plant organs are composed of three tissue systems: dermal, vascular, and ground
4. Plant tissues
are composed of three basic cell types: parenchyma, collenchyma,
and sclerenchyma
5. Meristems
generate cells for new organs throughout the lifetime of a plant
A plants continuous growth and development depend on processes that shape organs and generate specific patterns of specialized cells and tissues within these organs.
Growth is the irreversible increase in mass that results from cell division and cell expansion.
Development is the sum of all the changes that progressively elaborate an organisms body.
Most plants demonstrate indeterminate growth, growing as long as the plant lives.
In contrast, most animals and certain plant organs, such as flowers and leaves, undergo determinate growth, ceasing to grow after they reach a certain size.
Indeterminate growth does not mean immortality.
Annual plants complete their life cycle - from germination through flowering and seed production to death - in a single year or less.
Many wildflowers and important food crops, such as cereals and legumes, are annuals.
The life of a biennial plant spans two years.
Often, there is an intervening cold period between the vegetative growth season and the flowering season.
Plants that live many years, including trees, shrubs, and some grasses, are perennials.
These often die not from old age, but from an infection or some environmental trauma.
A plant is capable of indeterminate growth because it has perpetually embryonic tissues called meristems in its regions of growth.
These cells divide to generate additional cells, some of which remain in the meristematic region while others become specialized and incorporated into the tissues and organs of the growing plant.
Cells that remain as wellsprings of new cells in the meristem are called initials.
Those that are displaced from the meristem, derivatives, continue to divide for some time until the cells they produce begin to specialize within developing tissues.
Apical meristems, located at the tips of roots and in the buds of shoots, supply cells for the plant to grow in length.
This elongation, primary growth, enables roots to ramify through the soil and shoots to extend their exposure to light and carbon dioxide.
Woody plants also show secondary growth, progressive thickening of roots and shoots.
Secondary growth is the product of lateral meristems, cylinders of dividing cells extending along the length of roots and shoots.
One lateral meristem replaces the epidermis with bark and a second adds layers of vascular tissue.
5. You will only be
responsible for details on leaf anatomy
The leaf epidermis is composed of cells tightly locked together like pieces of a puzzle.
The leaf epidermis is a first line of defense against physical damage and pathogenic organisms and the waxy cuticle is a barrier to water loss from the plant.
The epidermal barrier is interrupted only by the stomata, tiny pores flanked by specialized epidermal cells called guard cells.
Each stoma is a gap between a pair of guard cells.
The stomata allow gas exchange between the surrounding air and the photosynthetic cells inside the leaf.
They are also the major avenue of evaporative
water loss from the
plant - a process called transpiration.
The ground tissue of the leaf, the mesophyll, is sandwiched between the upper and lower epidermis.
It consists mainly of parenchyma cells equipped with chloroplasts and specialized for photosynthesis.
In many dicots, a level or more of columnar palisade parenchyma lies over spongy parenchyma.
Carbon dioxide and oxygen circulate through the labyrinth of air spaces around the irregularly spaced cells.
The air spaces are particularly large near stomata, where gas exchange with the outside air occurs.
The vascular tissue of a leaf is continuous with the xylem and phloem of the stem.
Leaf traces, branches of vascular bundles in the stem, pass through petioles and into leaves.
Within a leaf, veins subdivide repeatedly and branch throughout the mesophyll.
The xylem brings water and minerals to the photosynthetic tissues and the phloem carries its sugars and other organic products to other parts of the plant.
The vascular infrastructure also reinforces the shape of the leaf.
6. You need to
understand a bit about secondary growth: Lateral meristems add girth by
producing secondary vascular tissue and periderm (see
Figs. 35.10 and 35.19)