I.  Sachs (1875)

• A.  "  In Phylogenetic Development of higher plants from simpler multicellular forms, a destinction gradually arose between outer layer of cells and internal mass of tissue.  The latter ultimately became differentitated into internal strands of cells surrounded by fundamental tissue."
• B.  Plant organs consist of three principle systems of tissues:
•     1.  Epidermal System = Epidermis and Cork Layers
•     2.  Fascicular System =  Xylem and Phloem
•     3.  Fundamental Tissue System = masses of tissue which remain after formation of 1 and 2
• C.  Advantages
•     1.  Simple to understand
•     2.  Practical to apply
• D.  Disadvantages
•     1.  Indefinite physiological and morphological nature of fundamental tissue

II.  Haberlandt (1914)

• A.  "The Principle Function should be sole guide in designating any specific tissue system."
•     1.  Principle Function is defined as that form of physiological activity with which its most obvious and important anatomical features are correlated.
• B.  Original Functional Tissue Systems
•     1.  Meristematic Tissues
•     2.  Dermal System
•     3.  Mechanical System
•     4.  Absorbing System
•     5.  Photosynthetic System
•     6.  Vascular or Conducting System
•     7.  Storage System
•     8.  Aerating or Ventilating System
•     9.  Secretory and Excretory Systems
•     10.  Motor System
•     11.  Sensory System
•     12.  Stimulus-transmitting System
• C.  Advantages
•     1.  Provides the most broadests and most natural of all systems of tissue classification since plant body is regarded not merely as a more or less complex aggregate of formal elements but also as a living organism, composed of a number of functional units engaged in a corresponding number of physiological activities, which all contribute to the safety and welfare of the whole.
• D.  Disadvantages
•     1.  Many tissues have multiple functions so principle and subsidiary functions are arbitrary
•     2.  Disregards origin of tissues since homology of tissues is ignored, the system of classification being based on analogy of function

SUPPORTIVE SYSTEMS

HOW DO PLANTS SUPPORT THEIR OWN WEIGHT UNDER THE FORCE OF GRAVITY AND OTHER EXTERNAL FORCES LIKE WIND, SNOW, ANIMALS, SOIL, AND WATER?

I.  Major forces acting on the idealized plant body

• A.  Aerial Parts (Stem and Leaves)
•     1.  Longitudinal compression due to weight of plant
•     2.  Bending due to wind, water, and animals establishes regions of tension and compression
•     3.  Shear due to wind action
• B.  Subterranian Parts (Roots)
•     1.  Longitudinal tension due to movement of aerial pats in relation to subterranean anchoring parts
•      2. Radial compression due to surrounding water and soil

II.  Basic Useful Physical Concepts that are useful to understanding Plant Supportive System and Human Engineered Constructions

• A.  Stress = Internal Resistance to an External Force
•     1. f = P/A or P = fA or A = P/f
•         where
•         f = unit stress (grams/mm^2)
•         P = external force or load (Newtons = grams)
•         A = cross section of structure under stress
•     2.  Tension = tensile forces tend to stretch a structure
•     3.  Compression = compressive forces tend to shorten a structure
•     4.  Shear = shearing forces tend to make parts of a structure slide past each other
•     5.  Torsion = torsional forces tend to twist a structure
• B.  Strain = Accompanying change in shape or size of a structure under an external force
•     1.  s = unit strain = e/l (mm/mm -> unitless)
•             where
•             e = total deformation (delta l)
•              l = initial length of structure
• C.  Hooke's Law (Robert Hooke using clock springs)
•     1.  Strains are directly proportional to Stresses
•     2.  Elastic Limit = unit stress beyond which the strain increases in a faster ratio than the applied force
•     3.  Yield Point = unit stress where strain increases without any increase in applied force
• D.  Modulus of Elasticity (Youngs Modulus) Indicates degree of stiffness of a material
•     1.  E = f/s
•         = unit stress divided by unit strain,
•         determined below Elastic Limit (grams/mm^2)
•      2.  E = f/s = (P/A) / (e/l) = (P/A)(l/e) or e = (Pl)/(AE)

ANALOGIES CAN BE DRAWN BETWEEN ENGINEERED CONSTRUCTIONS AND STRUCTURAL TISSUE SYSTEMS OF PLANTS

I.  Tensile Structures

•     A.  Buckminister Fuller's Geodesic Dome Constructions represent a solution to enclosing a volume using minimum materials in a structurally sound manner.
•         1.  Lattice work of regular poygons held under tension
•     B.  Parenchyma tissue represents a similar solution
•         1.  Net work of three dimensional polygons consisting of minimal material (middle lamella + primary cell wall)
•         2.  Each individual cell under tension via internal tugor pressure exerted on cell wall network

•     A.  Human constructions like the largest "H" or "I" beam bridge in the world (Wroclaw, Poland)
•         1. Stress is proportional to distance from the neutral axis
•         2. Bulk of the strongest material located at Flange regions of the beam
•         3. Interconnecting Webbing material is made of less or weaker material
•         4.  Conserves material while maximizing strength
•     B.  Pattern of arrangement of different cell types  within primary stem analogous to H beam construction
•         1.  Flange regions consist of cells with thick primary cell walls (collenchyma) or stiff secondary cell walls (fibers,      sclereids,tracheids, vessel elements)
•         2.  Interconnecting Webbing consists of less strong parenchyma cells
•         3.  Maximizes strength against bending while minimizing material

•     A.  Composite material that consists of network of structurally strong material embedding in weaker or thinner material
•         1.  Resistance to Shear Forces with minimal material
•     B.  Leaf lamina consist of network of structurally strong cells (xylem, fibrous bundle sheaths, and sclereids) embedding in weaker parenchyma cells.
•        1.  Resistance to Shear Forces with minimal material

•     A.  Theoretically resistance to tension is simply proportional to the cross sectional area of the resistant elements in a structure
•         1.  If the resistant structures are widely dispersed throught a structure, the probability that a subset of these structures will bear most or all of the force at any instant in time increases
•         2.  When this happens the probability that the Elastic Limit of this subset of structures will be reached increases
•         3.  A strategy to stabalize structures against such failure in longitudinal tension is to minimize the dispersion of the resistant elements such as is done in suspension bridges like the Golden Gate Bridge in San Fransisco California.
•     B.  Pattern of arrangement of different cell types within primary root is analogous to suspension bridge cables
•         1.  Strong resistant cells (xylem, sclereids) are centrally located which stabalizes the root against failure in longitudinal tension.

•     A.  Stress within a hollow cylinder is inversely proportional to thickness of cylinder
•         1.  Optimal construction of structures designed to withstand large radial compression experienced at deep sea levels is to have a thick shell of very strong material (hull)
•     B.  Two regions analogous to the hull can be found in primary roots
•         1.  The Exodermis which underlies the epidermis and serves to protect the cortex parenchyma from radial compression
•         2.  The Endodermis which is located outside the pericycle and serves to protect the pericycle parenchyma and phloem from radial compression