WHAT MUST PLANTS PROTECT THEMSELVES FROM IN ORDER TO SURVIVE?
Dessication
Temperature Extremes
Excess Insolation, including Ultraviolet
Radiation
Disease
Herbivory
I. A common feature to the above is that they all relate to some external environmental factor interacting with internal living plant cells
A. A logical place to look for protective anatomical features is therefore at the interface between plant organs and the external environment, i.e. at the surface of the organ.
B. The Epidermis provides the first line of defense in the primary plant body
C. The Periderm or Bark provides the first line of defense in the secondary plant body
D. In addition to these continuous coverings, we also observe a higher order arrangement of specialized plant structures which together serve to provide protection to more vulnerable plant parts.
1. Protection against herbivory
a. Thorns = sclerified pointed stems
b. Spines = sclerified
pointed leaves
c. Prickles = sclerified pointed trichomes
2. Protection against extreme temperatures
a. Underground stems
1. Tubers
2. Corms with contractile roots
3. Bulbs with contractile
roots
b. Dessicated seeds
3. Protection against dessication
a. Bud scales
1. Sclerified
2. Tightly packed
3. Often with resin
b. Dense trichomes on young leaf primordia
II. Epidermis
A. Outermost periclinal cellwall is cutinized and covered with cuticle and wax layers
1. Highly specialized to inhibit gaseous exchange
2. Cuticle extremely
resistant to various decay processes
a. Often only plant part left in fossil record
b. Protects against airborne bacteria and fungi
3. Wax also decreases wettability of plant surface
B. Epidermis cytoplasm stores water
1. Provides water reserve to interior intercellular air spaces
2. Saturates water pool that does slowly diffuse through periclinal cell wall
C. Anticlinal cell walls relatively thin
1. When epidermal cells become plasmolyzed, these walls collapse in accordian-like manner
2. Maintains structural integrity of periclinal walls of epidermis
3. These walls can return to initial state if turgor pressure is restored
III. Periderm (Peri = about; derma = skin)
A. Effectively replaces the protective function of epidermis when this tissue is destroyed via expansion in girth
1. Cork cells heavily suberized, lignified, some with layers of wax -> effective barrier for gaseous diffusion
2. Cork of some species contains resins and tannins -> repell invasive organisms
3. Many long lived species develop extremely thick periderms that protect stems against heat death in cases where fire burns rapidly through understory and accumulated litter.
B. Develops in association with leaf and branch abscission
C. Develops around areas of diseased or dead tissues with plant
D. Develops beneath surface of surface wounds
E. Terminology
| Bark = everything outside vascular cambium | Rhydtidome = Outer Bark = everything outside innermost Phellogen | Periderm |
| Secondary Phloem | Periderm | Phelloderm |
| Primary Phloem | May include Cortex | Phellogen |
| Cortex | May include Primary Phloem | Phellem |
| Periderm | May include Secondary Phloem |
F. Distinction needs to be made between
Spatial Origin of Initial and Subsequent Phellogens because of spatial
discontinuity of these in stems of many species
| Tissue forming Initial Phellogen | Tissue forming Subsequent Phellogens | Common to |
| Epidermis | Cortex Parenchyma | Nerium oleander |
| Epidermis + Cortex Parenchyma | Deeper Cortex Parenchyma | Pyrus sp. |
| Cortex Parenchyma | Phloem Parenchyma | Most Plants, Leguminosae, Pinaceae |
| Phloem Parenchyma | Deeper Phloem Parenchyma | Vitis, Phellodendron, Caryophyllaceae, Ericaceae |
G. Two types of Subsequent Phellogens
1. Ring like
a. Completely encircles the stem.
b. Usually associated with deep lying Initial Phellogen.
2. Shell or Scale like
a. Consists of discontinuous layers that curve toward outside.
b. Successive layers overlap older layers at margins.
c. Usually associated with more superficial Initial Phellogen
H. Phellogen arises from Pericycle Parenchyma or deep with Cortex Parenchyma in most roots
I. Most taxa develop Initial Periderm during first year after primary elongation has stopped.
1. Some species retain Initial Periderm for life or at least many years
a. Anticlinal division in phellogen keep pace with girth expansion
b. Fagus, Abies, Carpinus, Quercus
2. Most species replace Initial Periderm with Subsequent Phellogens on an annual basis
a. Cracking and furrowing of rhytidome becomes apparent over time
J. External appearance of Rhytidome determined by
1. Type of Subsequent Periderm
a. None or Ring -> +- Smooth
b. Shell or Scale
like -> Ridges & Furrows, or long strips
2. Relative amount and kinds of tissue isolated to outside by Subsequent Phellogens
a. Deeper the phellogen, the more furrowed the bark appears
b. Inclusion of bast fibers can give rise to net like patterns
c. If species has no bast fibers, the outer bark breaks up into individual
scale like units
3. Relative persistance of phellem
a. Some species slough off phellem on annual basis via thin walled phellem, phelloids(non-suberized phellem like cells), or underlying parenchyma -> thin scales or plates
b. Other species retain phellem for many years -> thick furrows and ridges or fibrous
K. Monocots rarely form Periderms but some develop analogous tissues via
1. Epidermis becoming extremely hard (Calamus)
2. Suberization of Ground Parenchyma (Poaceae, Typha)
3. Formation of Storied Cork (Dracaena, Yucca)
a. Outer Ground Parenchyma undergoes periclinal divisions to form
4-8 cell layers which become suberized
b. Subsequently deeper regions of Ground Parenchyma do the same thing
c. No recognizable Phellogen in these species
