The Plant Body – Tissues and Organs

Cooksonia – 408 MYA

Plant Tissues

• 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

• Ground tissue system - the inner supportive tissues of the plant - pith

Plant Meristems and Growth

• Primary growth is an increase in length – it occurs at apical meristems

• Secondary growth is an increase in plant diameter – it occurs at lateral meristems – in particular the vascular cambium and cork cambium

Dermal Tissues

• Make up outermost tissue layer of plants

• In young plants, it consists of a single layer of cells – the epidermis – that may secrete cutin to make protective wax layer of cuticle

• May have hairs or trichomes

Trichomes

Stomata

• Scattered through the leaf epidermis are openings called stomata that allow the plant to breathe

• The opening (pore) is surrounded by two guard cells

Periderm

• In woody plants the epidermis cracks and splits and is replaced by periderm which is formed by the cork cambium

• the periderm consists of cork cambium, cork cells and some other cells – it is the bark of mature trees -

• cork is mostly dead cells with cell walls containing much suberin

Periderm

Ground Tissues

• Ground tissue makes up most of the tissues in herbaceous plants• There are several ground cell types which perform a variety of

functions• Parenchyma cells – very diverse – often loosely arranged, main

location of photosynthesis and storage in leaves• Collenchyma cells – main support tissue in young plant stems –

found in leaves, stems and petals – usually with thickened corners of cell walls

• Sclerenchyma cells – can be either fibers or sclerids – fibers provide support but are dead at maturity – thick secondary cell walls; sclerids provide support as well

Tissues in an herbaceous stem

Mesophyll cellsare parenchyma

Parenchyma and Collenchyma

Sclerenchyma - Sclerids

Yucca leaf basket – sclerenchyma fibers

Vascular Tissues

• Vascular tissues are responsible for transporting material through the plant body

• Xylem cells move water and nutrients from roots to rest of plant

• Phloem cells move carbohydrates and other photosynthetic products from leaves to rest of plant

Xylem

• Xylem is dead at maturity and transports water essentially through a hollow tube - angiosperms have tracheary cells are called vessels which tend to have flattened ends, angiosperms also have tracheids

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

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

Xylem –VesselsandTracheids

Phloem

• 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

Phloem – sieve elements

The Roots

• Roots make up most of the underground portion of the plant

• Roots anchor plant in soil

• Roots absorb water and nutrients

• Roots serve as storage organs – especially storage of starch

Hornbeam Roots

Carrots – Storage Root

Carrot Flowers – Wild Carrot aka Queen Anne’s Lace

Fibrous Tap

Extent of Root Systems

Size of Root Systems• The most well studied root system was for a 4

month old rye plant - its roots occupied a volume of 6 liters

• When measured the total surface area of the root system, including root hairs was 639 m2, or 130 times the surface of the shoot

• It had approximately 14 billion root hairs with an absorbing surface of 401 m2 - if laid end to end, they would extend over 10,000 km

Size of Root Systems

• Deepest known roots – desert mesquite shrub roots down to 53.3 m

• Tamarisk and Acacia trees – roots to 30 m deep

• Herbaceous Alfalfa – roots to 6 m deep

Root Growth

• The growth of most roots is continuous process that only stops under adverse conditions such as drought or low temperature

• During their growth through the soil, roots follow the path of least resistance and frequently follow spaces left by earlier roots which have died and decayed

Mango Tree Root System

More Root Growth

• The tip of the root is covered by a root cap - a mass of cells which protects the apical meristem as it pushes through the soil

• As the root pushes through the soil, cells of the root cap are sloughed off from the margins - they are replaced by new growth of cells at the center of the root cap

• The sloughed off cells and growing root tip are covered by a slimy sheath called the mucigel which lubricates the root as it passes through the soil

Rhizosphere and Mucigel

• The mucigel provides an environment favorable for growth of beneficial bacteria

• The rhizosphere is formed of the mucigel, root hairs, sloughed off root cap cells and various microorganisms

Mucigel Sheath

Root Absorption

• Epidermis in young roots absorbs water and minerals and this is facilitated by root hairs - tubular extensions of the epidermal cells

• Some roots have a thin cuticle covering the epidermis; other roots have epidermal cells containing suberin - even so, the epidermal cells offer little resistence to the entrance of water and nutrients

• The innermost layer of the cortex is compact and lacks air spaces - this is the endodermis - it has Casparian strips which are bands of suberin between cells that prevent the passage of water and air - thus in endodermis, all substances must pass through cells