Plant Structure & Growth

Chapter 35

Plant structure is affected by genes and the environment

Plants have three basic organs

• Roots– Anchor plant in soil– Absorb water,

minerals– Store food

• Stems– Connect leaves

• Leaves– photosynthesis

Roots

Modified shoots

Stolons of strawberry Rhizomes

Tubers (Potato) Bulb (Onion)

Leaf anatomy

Modified leaves

Plant Tissue Systems• All 3 tissues are present in all

organs

• Dermal (epidermis)– Single layer– Protective, outer layer

• Vascular– Xylem: water transport

• Tracheids & vessel elements– Phloem: food transport

• Ground - neither dermal nor vascular– Pith: internal to vascular tissue– Cortex: external to vascular

tissue

Water-conducting cells of xylem

Food-conducting cells of the phloem

Plant Cell Types• Parenchyma

– Thin & flexible primary cell walls– “typical cells” – least specialized– Perform most metabolic processes of cell– Usually no secondary walls

• Collenchyma– Thicker primary walls (unevenly thickened)– Support parts of plant that are growing– No secondary walls

• Sclerenchyma– Thick secondary walls– Specialized for support– Two types

• Fiber - long• Sclereids – shorter, harder

3 major categories of plant cells

Growth @ Meristems

• Meristems– produce new cells

• Apical meristems– Located at root and shoot

tips– Growth increases plant

length (primary growth)

• Lateral meristems– Replaces epidermis with

secondary dermal tissue (i.e. bark)

– Secondary growth

Primary Growth

• Elongates roots and shoots

• Apical meristems produce new cells– Give rise to all tissue types– In roots: near root tip– In shoots: near terminal bud

Primary growth of a root

The terminal bud and primary growth of a shoot

Secondary Growth

• Fattens up the plant

• Produces secondary vascular tissue– Vascular cambium develops from parenchyma

• Produces secondary xylem & phloem

• Produces periderm– Cork cambium gives rise to periderm– Protective covering– Consists of cork cambium + cork cells produced

• Bark = Periderm + secondary phloem

Anatomy of a tree trunk

Anatomy of a three-year-old stem

Secondary growth of a stem

Summary of Primary & Secondary Growth in a Woody Stem

Monocots & Dicots

Organization of primary tissues in young stems

Plant Development

• Growth– Cell division – Cell expansion

• Morphogenesis– Development of body form

and organization (new leaves forming from meristems)

• Differentiation– New cells develop into

specialized cells– Gene expression– Cell location

The plane and symmetry of cell division influence development of form

The preprophase band and the plane of cell division

The orientation of plant cell expansion

Establishment of axial polarity

First cell division was asymmetrical

First cell division was symmetrical

ABC hypothesis for the functioning of organ identity genes in flower development

Phase change in the shoot system of Eucalyptus

Phase change: internal or environmental cue may cause plant to switch from one developmental phase to another (i.e. juvenile leaves to mature leaves)