BIO204 Notes Sept.8/2011
Chapter 36: Plant Form and Function
The vascular plant body consists of (1) a root system that anchors the individual and absorbs water and key ions, and (2) a
shoot system that absorbs carbon dioxide and sunlight. Both systems are dynamic—they grow and change throughout life.
Because body size and shape varies so much among species and individuals, plants are able to harvest light and other
resources in unique ways.
Primary growth occurs when cells located at the tips of each root and shoot divide. Primary growth extends the body and
gives rise to three primary tissue systems. These tissue systems are specialized for protection, food production and storage,
In some species, secondary growth makes roots and shoots widen throughout life. Secondary growth occurs when cells near
the perimeter of a root or shoot divide. Secondary growth adds transport tissue and provides additional structural support.
36.1 Plant form: Themes with many variations
For photosynthesis to occur, plants need large amounts of light and carbon dioxide and a small amount
of water as an electron source (Figure 36.1).
They also need large amounts of water to fill their cells and maintain them at normal volume and
Obtain nitrogen, phosphorous, potassium, magnesium, and other nutrients to synthesize the
macromolecules needed to build and run cells.
The root system and the shoot system are the two basic systems
for acquiring and transporting nutrients (Figure 36.2).
The root system is below ground, anchors the plant, and takes in
water and nutrients from the soil.
The shoot system is above ground and harvests light and carbon
dioxide from the atmosphere. Both systems connected by vascular tissue,
which allows transport between them.
Most root and shoot systems have the same general structures
(Figure 36.3). The organization of the vascular tissue is not identical in all
plants, and the morphology of root and shoot systems and leaves varies
The Root system
Many root systems have a vertical section called a taproot and
numerous lateral roots that run more or less horizontally.
The root system functions to anchor the plant in the soil absorb
water and ions form the soil, conduct water and selected ions to the shoot,
and store material produced in the shoot for later use. Morphological Diversity
Root systems have similar functions in all plants but can be highly diverse is structure. Figure 36.4
shows the diversity of root systems in prairie plants.
The diversity of root systems observed in prairie plants has two important consequences: (1) an array of
plants can coexist in the same area with less competition for soil resources; and (2) most individuals can
survive intense water stress during drought years.
Roots show a great deal of phenotypic plasticity, meaning that they are changeable depending on
Even genetically identical individuals will have very different looking root systems if they are growing
in different types of environments.
Phenotypic plasticity is important because plants grow throughout their lives. Root actively grow into
areas of soil where resources are abundant, and do not grow or die back in areas where resources are
Adventitious roots are roots that develop from the shoot system instead of the root system.
Roots can also be modified for functions other than anchoring and absorbing water and ions from the
soil. Specialized lateral roots function in gas exchange.
Among individuals, the overall size and shape of the root system is variable and dynamic .
The Shoot system
The shoot system consists of one or more stems, which are vertical aboveground structures.
Each stem consists of nodes, where leaves are attached, and internodes—segments between nodes.
A leaf is an appendage that projects from the stem laterally. Leaves usually function as a photosynthetic
The nodes where leaves attach to the stem are the site of axillary buds. The axillary bud may develop
into a branch—a lateral extension of the shoot system.
At the tip of each stem is an apical bud, where growth occurs that extends the length of the stem or
Once a leaf, node, or internode forms, it does not increase much in size over time. Instead, plants grow
by adding more parts rather than increasing the size of each part.
The size and shape of shoot systems can vary greatly among species. Plant form can vary as a function
of branch angle and internode length (Figure 36.7).
Variation in size and shape of the shoot system allows plants of different species to harvest light at
different locations and thus minimize competition. It also allows them to thrive in a wide array of
habitats. Silverswords diverse in size, shape and growth habit. Can be rosettes, thick woody or form cushions or
The diversity of shoot system a suite of adaptations for harversting light and CO in 2arious env.
Shoots respond to variation in light availability.
Because the shoot system continues to grow through the lifetime of the plant, it can respond to changes
in environmental conditions.
Modified stems are common. Examples are the stems of
cacti, stolons (stems that run over the soil
surface), rhizomes (stems that grow horizontally
underground), tubers (rhizomes modified to store
carbohydrates), and thorns (stems that protect the plant (Figure
In most plant species, the vast majority of photosynthesis
occurs in leaves.
The external anatomy of a typical leaf consists of the blade and
the petiole (Figure 36.11a).
Leaves usually have an easily recognizable blade, but these may vary in size and shape.
Large SA = more loss of water so leaves adapted
The arrangement of leaves on a stem can also vary.
Plants thus have dramatically different ways to arrange their leaves in space, presumably to maximize
the efficiency of light capture.
Although leaves do not grow continuously, they do exhibit phenotypic plasticity.
One example is that oak trees have large, broad shade leaves (high SA which max. absorption of rare
photons) and smaller sun leaves(reduce water loss where light abundant). Modified Leaves
Not all leaves function primarily in photosynthesis.
Other leaf functions include food or water storage, climbing, attraction of pollinators, trapping prey (for
carnivorous plants), and collecting soil (Figure 36.14).
Flowerpot plant used by ants has adventitious roots which absorbs nutrients from ant feces and other
material that accumulate inside.
36.2 Primary Growth
Plants grow continuously because they have meristems – populations of undifferentiated cells that
retain the ability to undergo mitosis and produce new cells.
Apical meristems located at tip of each root and shoot. As cell # increases in response to cell divison and
as cell size increases during differentiation, the meristems at each root and shoot tip extend the plant
body outward allowing exploring extra space. Process is called primary growth. It increases length if
roots and shoots.
Cell derived from apical meristems form primary plant body
How do apical meristems produce the primary plant body?
Apical meristems give rise to three distinct populations of cells that give rise to three major tissue
systems of the plant (Figure