The Plant Body

Fun plant facts

• There are over 270,000 species of plant - they differ from each other in light gathering shoots and water/nutrient gathering roots, but they also differ from each other in terms of biochemistry

• Because plants can’t move they have difficulty reproducing and evading predators

• Plants produce many different chemicals to prevent or at least minimize animal predators - many of these defensive chemicals are used by us for medicines – 80% of all people get their medicines directly from plants - 25% of our prescription medicines come from plant chemicals

Commonly used and abused chemicals substances from plants

• caffeine in coffee• theophylline in tea• theobromine in chocolate• nicotine in tobacco• cocaine from coca plant leaves• THC in Cannabis• and many other narcotic and psychoactive

substances

Cooksonia – 408 MYA

Basic plant structures

Plant Tissue Types

• Meristematic tissue - site of growth in plant; origin of the other tissue types

• apical meristems - site of primary growth; lateral meristems - site of secondary growth

• Dermal tissue system - the outer protective covering of the plant

• Vascular tissue system - comprises the xylem and phloem - it is embedded within the ground tissue system – xylem forms wood in woody plants

• Ground tissue system - the inner supportive tissues of the plant – pith in herbaceous plants

Primary Plant Growth

• Primary growth originates in the apical meristems and results in increases in length - tissues originating from primary growth make up the primary plant body (primary xylem, primary phloem, etc) - many vascular plants consist entirely of primary tissues

Secondary Plant Growth

• Secondary growth originates in the lateral meristems and results in increases in width - there are two lateral meristems

• Vascular cambium which produces secondary vascular tissues - secondary xylem to the inside and secondary phloem to the outside

• Cork cambium is outside the vascular cambium - it forms periderm, which is made of cork tissue - the periderm replaces the epidermis as the dermal tissue system of the plant

Vascular Tissues• Secondary xylem is dead at maturity and transports water

essentially through a hollow tube - angiosperms have tracheary cells are called vessels and tend to have flattened ends, as well as tracheids

• in gymnosperms the tracheary cells are called tracheids and usually sharply tapered

• eventually the xylem becomes full of sap and is no longer used for water transport, then it functions in support and forms wood

• Phloem cells are called sieve tube elements because of the sieve like plates at the end of the cells - they are alive at maturity but are crushed as the plant grows in diameter and must be continually replaced

• Some sieve cells have companion cells which govern transport of material through the sieve

Xylem –VesselsandTracheids

Phloem – sieve elements

Plant growth

• Plant material originating from secondary growth forms the secondary plant body

• Secondary growth appeared in the Devonian about 380 MYA

• Many plants grow continuously throughout their life and have no final particular size or shape - this is called indeterminate growth

• Many plant parts such as leaves, flowers and fruit do have a characteristic final shape and size and stop growing at that size - that is called determinate growth

Garlic Roots

Root vegetables

Fibrous Tap

Varieties of Root Systems

Soil root traces

Lateral growth of a willow root

Back to the shoot system

Plant Embryo

Apical Dominance

• Usually the growing terminal bud inhibits the development of the lateral buds, a phenomenon known as apical dominance - as the distance between the shoot tip and lateral buds, the influence of the apical meristem lessens and the lateral buds proceed with their development

Apical Dominance

Bud Pinching

Stem FunctionThe two main functions of the stems are conduction

and support• Conduction involves moving substances

manufactured in the leaves through the phloem to other parts of the plant including developing leaves, stems, roots, developing flowers, seeds and fruits and the xylem carries water from the roots to the leaves, where water is transpired

• Support involves holding the plant off the ground - supporting the principal photosynthetic organs of the plant (the leaves) as well as flowers, seeds and fruits

Stem Growth

• The organization of the apical meristem of the stem is more complex than is the organization of the apical meristem of the root

• The apical meristem adds cells to the plant body and forms leaf primordia and bud primordia that develop into lateral branches

• The apical meristem of a stem lacks a protective cover like the root cap of roots

Stem Growth cont’d

• Protoderm always originates from the outermost meristem cell layer

• Procambium and part of the ground meristem (which will form the cortex and sometimes part of the pith) form from the peripheral meristem

• The rest of the ground meristem (which forms some or all of the pith) is formed by the pith meristem

More Stem Growth

• Usually the meristematic activity causing the elongation of the internodes is most intense at the base of the developing internodes - if elongation of the internodes occurs over a long period, the meristematic base of the internode may be called an intercalary meristem (a meristematic region between two highly differentiated regions) – intercalary meristems are very important in the growth of grasses and grass-like plants

Three types of primary structure

1. In some conifers and dicots, the narrow, elongated procambial cells (and consequently the primary vascular tissues that develop from them) appear as a more or less continuous hollow cylinder within the ground tissue - the outer region of the ground tissue is called the cortex and the inner region is the pith

Basswood stem cross-section

Three types of primary structure

2. In other conifers and dicots, the primary vascular tissues develop as a cylinder of discrete strands separated from one another by ground tissue

• The ground tissue separating the procambial strands (and later mature vascular bundles) is continuous with cortex and pith and is called the interfascicular parenchyma (between the bundles)

• The interfascicular regions are often called pith rays

Sunflower stem cross-section

Three types of primary structure

3. In most monocots and some herbaceous dicots, the arrangement of procambial strands and vascular bundles is more complex - vascular tissues do not appear as a single ring, but instead develop as more than one ring or are scattered throughout the ground tissue - here ground tissue cannot be distinguished as pith and cortex - often called pith

Corn stem cross-section

Twig structure

Leaf Traces• At each node, one or more

vascular bundles diverge from the cylinder of strands in the stem, cross the cortex and enter the leaf or leaves attached at that node

• The extensions from the vascular system in the stem toward the leaves are called the leaf traces

• The wide gaps or regions of ground tissue above the level where leaf traces diverge toward the leaves are called leaf gaps

Plant habitat and leaf structure

• mesophytes - plants that require abundant soil moisture and a fairly humid environment - the most common plants - typically have fairly well developed epidermis, especially on upper surface of leaf, stomata on both sides of leaf

• hydrophyte - plants that depend on a very abundant supply of water or which grow wholly or partially submerged in water - have thin epidermis, stomata only on upper surface

• xerophyte - plants adapted to arid habitats - very thick epidermis, stomata open to stomatal crypts with protective hairs

Dicot stomata

Red Oak Leaves

Shade Leaves Sun Leaves