Chapter 35: Plant Structure, Growth, and Development

3 Levels of Plant Body Organization 

• Multicellular organisms are more that just a cluster of cells

1. Cells: various types; differentiated to perform specific functions 

• Cells for photosynthesis, support, absorption, transport, etc.

2. Tissues: composed of cells with similar form and/or function

• Dermal (external)

  • Protects plants from environment, desiccation, diseases, parasites, etc.

• Vascular (transport)

  • Xylem and phloem 

• Ground (other)

3. Organs: organization of tissues to perform some function; two main systems of vegetative growth

• Root system: roots

• Shoot system stems and leaves

Root System Operates Below Ground

• Root functions: anchor, absorb minerals and water, store carbohydrates

• Two main root morphologies

  • Taproot: main vertical root, many lateral roots

    • Gymnosperms and most eudicots

  • Fibrous roots: no main root; lots of little branches

    • Monocots (e.g., grasses) and seedless vascular plants 

• Have root hairs: increase surface area for absorption

  • Extensions of root epithelial (dermal) cells

• Many different kinds of modified roots to serve specialized purposes 

  • Adventitious roots: root tissue emanating from stems or leaves (shoot system)

Shoot System Functions Above Ground 

• Shoot system: reproduction and photosynthesis (stems and leaves)

• Stems: grow to increase the above ground volume occupied by branching

  • Alternating node (branching points) and internodes

  • Primary growth via apical bud, with branched formed by axillary bunds (both have meristem cells)

• Apical dominance: axillary bud growth is inhibited by proximity of apical bud

  • Most plants exhibit this

  • Prevents plant from getting top heavy

Leaves are Part of the Shoot System

• Leaves: main organ of photosynthesis

  • Composed of blade and petiole

  • Monocots and eudicots: parallel veins (vascular tissue) vs netlike veins 

  • Simple vs compound leaves:

    • Defined by axillary bud

  • Conifer needles are leaves

  • Leave can be modified for support, protection, etc

• Flowers are part of the shoot system, but they are involved with reproductive growth as opposed to vegetative growth

5 Different Characteristics Plant Cell Types

• Different cell types recognized based upon these characteristics

  • Degree of development of cell wall

  • Whether they function alive or dead

  • Involved with vascular tissue or not

1. Parenchyma cells:

• 1° cell walls are thing

• Photosynthesis

• Storage

• Metabolic factories

• Differentiate into other cell types  

2. Collenchyma cells:

• Living support cells with unevenly thickened 1° cell walls

• Often around vascular tissue 

• Flexible support without restraining growth 

• Thicken in response to different stimuli 

3. Sclerenchyma 

• Support cells, dead at maturity

• With both 1° and 2° (extra thick) cell walls 

• Hard (have lignin)

• Fibers and sclereids made of this cell type 

• Makes nut and seed shells hard; why pear are gritty 

• Expected to be found in larger plants 

4. Xylem 

• Part of vascular system 

• Water conducting  cells

• Dead support cells with both 1° and 2° cell walls; lignified 

• Tracheids: long narrow cells; connect by pits lacking 2° cell walls

• Vessel elements: shorter, wider cells 

  • Open on the ends, opposed to tracheids

5. Phloem

• Part of vascular system

• sap/sugar conducting cells

• Living cells “conductive parenchyma cells” 

• Sieve cells: long narrow cells in seedless vascular plants & gymnosperms

• More complex sieve tubes in angiosperms

  • Sieve-tube elements: no nucleus, cytoskeleton, vacuoles, ribosomes for fast conductance, connected by sieve plates

  • Companion cells: non-conducting partner cell connected via plasmodesmata; has nucleus, etc. that functions for both cells

  • Companion cells: non-conducting partner cells connected via plasmodesmata, has nucleus, etc. that functions for both cells 

Growth is Different in Plants and Animals

• Vertebrate growth is generally determinate: growth limited to embryonic/juvenile phase

• Plant growth generally indeterminate: made up of embryonic, juvenile, and adult organs

  • Can keep growing & living until killed

• Meristems: perpetually embryonic tissue

  • Can differentiate into other cell types

  • What makes indeterminate growth possible 

• Not all plants live forever

  • Annuals: complete life cycle in 1 year

  • Perennials: can live indefinitely

Plants Need to Grow in Length and Girth

• Two main types of meristems: allow plants to both lengthen and increase the diameter of roots and shoots

• Apical meristems: responsible for primary growth 

  • Primary growth: increase in length 

  • Tips of roots and apical and axillary bud of shoots

  • Most growth in herbaceous plants 

• Primary plant body: all parts of the plant 

• Lateral meristems: cause secondary growth (girth) in woody plants

  • “Cylinder of cells” that extends through stems and roots

  • Vascular cambium: adds 2° vascular tissue

  • Cork cambium: replaces epidermis with periderm

• Secondary plant body: all parts of plant that are growing without increasing in length; woody parts

Tissues of Leaves

• Leaves are composed of the same 3 tissue types

• Dermal tissue: upper and lower epidermis

  • Stomata: openings in the epidermis that allow air to reach photosynthetic ground tissue; guard cells: open and close stomata 

• Ground tissue: mesophyll = photosynthetic parenchyma cells

  • Space between mesophyll cells allows diffusion of gases

• Vascular tissue: branches throughout mesophyll; continuous with stem vascular tissue

  • Surrounded by protective bundle sheath (parenchyma)

  • Supported by supported sclerenchyma fibers

How is Plant Development Controlled?

• Plants grow in length and girth by developing from meristems; three important developmental processes

  • Growth: cells need to grow (elongate) as well as divide

  • Morphogenesis: specific tissues/organs must develop at specific places (pattern formation)

  • Differentiation: cells change from unspecialized to meristematic cells to specific specialized types

  • We know a lot of what we know by studying arabidopsis 

    • Model laboratory plant, whole genome sequence 

    • Learn the functions of different genes by creating mutants