Tissues Primary Structures & Introduction to Development

In the beginning….or in a galaxy far, far, away!
 Plants began living on land 420 million years ago.
 Challenges of terrestrial life led to evolution of distinct, specialized tissues and “organs”.
 All flowering plants have leaves, stems, and roots.
− These parts can be modified and may not be easily recognizable.
− No species exists for which roots, stems, or leaves have been completely lost evolutionarily.
 Flowering plants are classified as the Division Magnoliophyta, informally as angiosperms.
 You may have noticed that plants are placed in divisions and not phyla (phylum)
 It’s the same thing!!!!!!

Basic Types of Cells & Tissues
 Stems, leaves, and roots all share a basic, simple organization.
 All plant cells belong to just three classes based on the nature of their cell wall.
− Parenchyma
− Collenchyma
− Sclerenchyma

Parenchyma
• Parenchyma cells have only thin primary walls.
• A mass of parenchyma cells forms parenchyma tissue.
− Active metabolically.
− Most remain alive after they mature.
• Special parenchyma
− Chlorenchyma
− Glandular cells
− Transfer cells
− Phloem

Specialized Parenchyma
 Chlorenchyma cells are photosynthetic parenchyma cells.
− Thin walls allow light and carbon dioxide to pass through to the chloroplast.
 Pigmented cells are parenchyma.
− Thin walls of parenchyma cells also allow pigments to be seen.
Glandular cells secrete
− Nectar
− Fragrances
− Mucilage
− Resins
− Oils

Transfer cells mediate short-distance transport of material.
− They have a large, extensive plasma membrane with numerous molecular pumps.

Phloem (flowing sugars and nutrients) is parenchyma tissue that conducts nutrients over long distances.

Some parenchyma cells function by dying at maturity to open areas

Parenchyma cells are relatively inexpensive to build.

− Little glucose is expended in constructing such thin walls.

− Most leaves are soft, composed almost entirely of parenchyma.

− They are not very expensive metabolically

Collenchyma
 Collenchyma cells have a thin primary wall that becomes thickened in other areas.
 This allows plasticity.
 Collenchyma tends to exist:
− Beneath the epidermis
− Supporting vascular bundles

Sclerenchyma
 Sclerenchyma has a primary wall and a thick secondary wall that is usually lignified (legmen~primary componet that makes wood)
 These walls are elastic.
 Sclerenchyma supports the plant by its strength.
 Usually dead at maturity
 Two types:
− Mechanical
− Conductive

Mechanical sclerenchyma
− Fibers are long and flexible.
− Sclereids are short, isodiametric (cuboidal), inflexible, and brittle

Sclerenchyma
 Conducting sclerenchyma transports water.
− Tracheary elements of the xylem (sugar flows Phylum, water and minerals move through Xylem, making up a plant vascular system)
 Small, plasmodesmata-rich areas must remain free of the secondary wall.
− These become narrow pits in the secondary wall.
− Two pits are called pit-pairs.

Internal Organization of Stems: Primary Tissues
 Epidermis is the outermost surface of an herbaceous stem.
− A single layer of parenchyma cells.
− All interchange of material between a plant and its environment occurs by means of its epidermis.
− Functions in protection and preventing water loss.

Primary Tissues: Epidermis
 Outer tangential walls are coated with waterproof cutin.
 It builds up as a layer called the cuticle.
 Under dry conditions a wax layer may be added external to the cuticle.
 The cuticle prevents desiccation but it also prevents gas exchange.
 This critical function is accomplished by stomata.
− Guard cells
− Stomatal pore
 Guard cells swell by absorbing water.
− The pore between them opens, permitting entry of carbon dioxide and exit of oxygen.
 Water is lost through stomata.
− Guard cells regulate when the pores are open/closed.
− Remain closed after sunset or during periods of water stress.
− Stomata Animation

 Some epidermal cells elongate outward and become trichomes (hairs).
 Protective roles
− Deter herbivory.
− Minimize water loss.
− Protect for over exposure to sunlight.

Primary Tissues: Cortex
 Cortex is interior to the epidermis.
− Often homogenous, composed of photosynthetic parenchyma and sometimes collenchyma.
− Cells are typically tightly fitted, but some plants have a cortex of aerenchyma, loosely packed with large intercellular air spaces.

Primary Tissues: Vascular Tissues
 Vascular tissues are responsible for the conduction of materials throughout the plant.
 Two types of vascular tissues occur in plants.
− Xylem conducts water and minerals.
− Phloem distributes sugars and minerals.
 Xylem is dead and hollow at maturity.
 Phloem remains alive at maturity.

Vascular Tissues: Xylem
Xylem consists of tracheids and vessel elements.
Collectively referred to as tracheary elements

—> Strength of cells due to the secondary cell walls.
Annular thickenings, Helical thickening, Scalariform thickening, Reticulate thickening

Vascular Tissues: Xylem
 The most derived and strongest tracheary elements have circular bordered pits.
− The primary wall is underlain by the secondary wall.
− The pits are weak points in the wall.
− The weakness is reduced by a border of extra wall material.

Vascular Tissues: Xylem
 Water moves between tracheids through pit membranes.
 Vessel elements provide a way to move water with less friction.
− Perforations form between vertically stacked vessel elements.
− A stack of vessel elements is called a vessel.

Vascular Tissues: Xylem
 All plants with vascular tissue have tracheids.
− Conifers have only tracheids.
− Also found in leaf veins of flowering plants.
 Only flowering plants have vessel elements and tracheids.
− Perform long-distance water conduction in roots and stems.

Vascular Tissues: Phloem
 Phloem has two types of conducting cells.
− Sieve cells
− Sieve tube members
 The term “sieve element” refers to either.
 Develop from parenchyma cells that remain alive.
 Plasmodesmata enlarge to become sieve pores.
 The sieve pores aggregate in sieve areas.

Vascular Tissues: Phloem
 A sieve cell is similar in shaped to a tracheid.
− It is elongated and tapered.
 Sieve areas are distributed over its surface.
 This type is found in older fossils and in non-flowering vascular plants.

Vascular Tissues: Phloem
 Sieve tube members are similar to the vessel members of the xylem.
− Sieve plates on each end-wall.
− Align vertically to form a sieve tube.
− More effective flow of sap.
 All angiosperms have sieve tubes.
− None of the non-angiosperms have them.

Vascular Tissues: Phloem
 Nuclei of sieve elements degenerate.
− No complex metabolism without the nucleus.
− They are associated with neighboring cells that exert nuclear control.
 Sieve cells are associated with albuminous cells.
 Sieve tube members are controlled by companion cells.
 Companion cells are involved in loading sugars.
 They are often smaller than the accompanying conducting cell.
 They have a prominent nucleus and dense cytoplasm filled with ribosomes.

https://www.google.com/search?q=phloem+loading&client=firefox-b-1-d&tbm=vid&source=lnms&sa=X&ved=0ahUKEwj-sMC-sLbkAhXqna0KHYuvAlYQ_AUIDigC&biw=1408&bih=688&dpr=1.36

Organization of Vascular Tissues
 Xylem and phloem occur together in vascular bundles interior to the cortex.
− Arrangement of bundles differs between monocots and other angiosperms.
 Each bundle includes xylem and phloem running parallel to each other.

Organization of Vascular Tissues
 In eudicots, vascular bundles are arranged in one ring surrounding the pith.
− Pith is a central region of parenchyma similar to the cortex.
 In monocots, they are distributed as a complex network throughout the inner part of the stem.

Organization of Vascular Tissues
 The xylem of a vascular bundle is primary xylem.
 In addition to the tracheary elements there are:
− A large proportion of xylem parenchyma
− Xylem fibers
 The phloem of a vascular bundle is primary phloem.
 In addition to the sieve elements and their associate cells, there are
− Storage parenchyma
− Phloem fibers or sclerids

Stem Growth and Differentiation
 Stems grow longer by creating new cells at their tips.
− Regions known as apical meristems.
− Cells here retain their ability to divide.
− Expanding daughter cells push the apical meristem upward.
− Subapical meristems contain cells dividing and growing, producing cells for the region below.

Stem Growth and Differentiation
 In the subapical meristem, differentiation begins as some cells stop dividing.
 Some differentiate in protoxylem—the first xylem to appear.
− Protoxylem must be extensible while cells around them continue to elongate.
− This limits their 2nd wall thickenings to annular and helical.
 Some of the elongating cells will differentiate and mature into metaxylem.
− Since the cells around it have stopped elongating producing one of the stronger 2nd wall types is feasible.

Stem Growth and Differentiation
 A similar process occurs in the outer part of each vascular bundle.
− Exterior cells mature as protophloem.
− Cells closest to the metaxylem become metaphloem.
 Other tissues also differentiate in the subapical region.
− Epidermis
− Pith
− Cortex
Stem Growth and Differentiation
 Protoderm is epidermal cells that are in the early stages of differentiation.
 Young xylem and phloem cells are provascular tissues.
 The equivalent stages of pith and cortex are ground meristem.
 Primary tissues are produced by apical meristems.
 Primary growth is the growth and tissue formation that results from apical meristem activity.