Circulatory System in Plants: I

February 10, 2010

(pp. 718-735)

Summary Plant Circulation Figures

1. Introduction

      Quick review of diffusion
How do cells obtain gases, nutrients, food and water from the environment?  Eliminate wastes?
      Simple aquatic/marine organisms are in intimate contact with the aqueous environment.
      Gas, nutrient, food, water, waste exchange relatively simple and fast for:
                    unicellular organisms (e.g. Xanthidium)

Multicellularity appeared in fossil record ~ 900 million yr. ago.
       Does multicellularity present problems with gas, nutrient, food, water, waste exchange?
       Not if organism is small, simple, with high S:V
                    simple filaments (e.g. Oscillatoria)
                    thin sheet-like multicellular organisms (e.g. Ulva)
                    hollow or tubular shaped organisms (e.g. Jellies)

       Life on land began ~ 459 million yr ago.
       Does life on land present problems?
       Again, not if organism is small, simple, with high S:V and is limited to wet areas.
                    (e.g. liverwort)

      What if organism is large? With unfavorable S:V?  Will cell-to-cell diffusion allow cell far
              removed from the external environment access to gases, nutrients, food, water?

       Diffusion is too slow.  Need for a circulatory system is evident.

       How do large/terrestrial plants circulate substances to and from tissues?

2. Plant Anatomy

  Plants bearing vascular tissue include ferns, conifers, flowering plants

Stems:  Cross sections away from growing tip illustrate differentiation of tissues
                  Apical meristem gives rise to: protoderm
                                                                 ground meristem
procambial tissue

Procambial tissue matures into vascular cambium that gives rise to xylem and phloem.
Vascular tissues in bundles (XC dicot and monocot stems).

Xylem cells are dead when mature, hollow, cylindrical
                   vessel elements and tracheids

Phloem cells: sieve cells, sieve tube elements w/ companion cells.  Alive when mature.

Leaves: Shape - high S:V
                   Waxy cuticle prevents water loss
Leaf is supported by a skeleton of relatively rigid veins.
                    Leaf veins are bundles of xylem and phloem cells continuous with stem xylem/phloem.

Guard cells in pairs - opening between is stoma

Study Questions:
1) A theory in phytoplankton ecology predicts that phytoplankton communities in nutrient-poor environments are composed of unicellular species with very small cells, and those communities in nutrient-rich environments are composed of unicellular species with larger cells.  Why might this be true?
2) Trace the development of xylem and phloem tissue from the apical meristem.
3) Compare and contrast xylem and phloem cells.
4) What tissues are encountered in a radial path through a dicot stem with secondary growth?
5) What tissues are encountered in a cross sectional path through a leaf?

Key words: vascular tissue, apical meristem, protoderm, ground meristem, procambial tissue, xylem, phloem, vascular cambium, vessel elements, tracheids, sieve cells, sieve tube elements, epidermis, palisade mesophyll (parenchyma), spongy mesophyll (parenchyma), vascular bundles, bundle sheath, guard cells, stoma, sub-stomatal cavity, turgor