Plant Basics

Plants evolved from ancestors shared with all other organisms on planet Earth.

The diagram below shows how one might divide (classify) the organisms on our planet into major groupings.

 Extinct OrganismsExtant
Organisms
8 Kingdoms  2 Domains
         CyanobacteriaBacteriaProkarya
First              
Life               Eubacteria
           
           ArchaeaArchaea
      ↓ Endosymbiosis 
  N+        ArchezoansProtistaEukarya
   M+      
          C+     Dinoflagellates
         
        C-Ciliates
         
       C-   Zooflagellates
         
           CgEuglenoids
            
     Cr      Brown algaeChromista
            
        C-Water Molds
            
             Golden Algae
Neo-endosymbiosis      
          Diatoms
          
        Red AlgaeRhodophyta
          
Neo-endosymbiosis   Green AlgaePlantae
         
         Plants
      C-  
Cladogenesis = branching   FungiFungi
Anagenesis = extension   
         AnimalsAnimalia
         

In this cladogram, major groups (branches) have shared-derived characteristics (synapomorphies). Branches which share common ancestry are called sister groups. The characteristics two sister groups share are called plesiomorphies. It is clear that all living organisms are sister groups to each other and share certain pleisiomorphies. At the base of this cladogram all organisms share: cellular structure with cell membrane, DNA hereditary material coding for the structure of proteins, proteins which function in various ways including trans-membrane movement of solutes. These and more are the plesiomorphies.

The organisms whose descendents constitute kingdom Eubacteria separated off early in evolution and have specialized through evolution into what has been called domain Bacteria. These organisms include bacteria that are responsible for disease, others that produce medicines, others that fix nitrogen, and ancestral types (protobacteria) which were taken into eukaryotic cells (endosymbiosis) and evolved into the mitochondria and chloroplasts. The distinguishing derived characteristics, apomorphies include lack of histone proteins, distinctive promoter regions in the DNA, and distinctive RNA polmerases. This domain and kingdom includes the cyanobacteria (phylum Cyanophyta), once called blue-green algae.

The line of evolution from the universal ancestor evolved some early apomorphies during anagenesis: development of histone proteins, distinctive DNA promoters, and distinctive RNA polymerases. These features are conserved among all living organisms that evolved subsequently along this line; they are now plesiomorphies for the remaining sister groups. Derived characteristics that are shared by taxa are called synapomorphies. One branch after these developments specialized and produced descendants that are today known as kingdom Archaebacteria and domain Archaea. These organisms lack a nucleus and so are prokaryotic still, and they also lack chloroplasts and mitochondria.

The main line of evolution then produced a nuclear envelope to enclose the DNA. This innovation (apomorphy among all living organisms) is shared by all descendents from that point and so is a synapomorphy for those organisms considered eukaryotic. But it is a plesiomorphy when considering only the eukarytoic clade. Thus all subsequent new life forms in this line and its branches are eukaryotic (have a true nucleus) constituting domain Eukarya. Perhaps the most primitive eukaryotic group splitting out (cladogenesis) and evolving along its own branch is members of the kingdom Archezoa. An example archaezoan is Giardia, a tiny unicellular organism that is a parasite in mammals, particularly beaver. These can pass through older filtration systems for municipal river water supplies and then appear in tap water. When people drink this water some severe diarrhea and other symptoms can ensue...up to and including death of weaker persons. These organisms have a nucleus but lack any kind of mitochondrion or chloroplast. They are the "archetypal" eukaryote according to one hypothesis. However, if you have had organismal biology taught by me in recent years, you realize that the alternative hypothesis is that archezoans are advanced eukaryotes with apomorphic steps known as reversals. Which hypothesis is correct can only be resolved by further study...but parasites are famous for reductionism!

Sometime after the archaezoans split off from the main line of evolution, the ancient apomorphies, which are now plesiomorphies of all subsequent groups, include the endosymbiosis events. Ancient protobacteria were taken in by primitive eukaryote hosts as respiratory endosymbionts and evolved into mitochondria. Ancient cyanobacteria were taken in as photosynthetic endosymbionts and evolved into chloroplasts. Major organisms that evolved as a single branch early after these endosymbiosis events are now called kingdom Protista. Dinoflagellates (phylum Pyrrophyta) perhaps represent an early-departing twig on that branch. Ciliates and Zooflagellates lost the chloroplast and therefore have more animal-like characteristics. Euglenoids (phylum Euglenophyta), perhaps allied with Zooflagellates, departed along their own separate twig regaining a chloroplast by neo-endosymbiosis of a green alga.

Another eukaryotic branch developed when the original chloroplast was replaced by a secondary endosymbiosis (aka neo-endosymbiosis). These organisms, now called kingdom Chromista, share the synapomorphy of having a derived red algal endosymbiosis that evolved to replace the standard chloroplast. This derived Chromista chloroplast thus has characteristics that are shared with derived chloroplasts of red algae. The Chromista include Brown algae (phylum Phaeophyta), the water molds (phylum Oomycota), the golden algae (phylum Xanthophyta), and the diatoms (phylum Chrysophyta).

Yet another eukaryotic branch, evolving with the original chloroplast, diversified into the organisms known today as members of kingdom Plantae. One twig on this branch specialized to become the red algae (phylum Rhodophyta), which participated in the later endosymbiosis event that gave rise to kingdom Chromista. Rhodophyta are considered by most modern authors as constituting their own kingdom. One twig of kingdom Plantae became what are now known as green algae (phylum Chlorophyta). From this came another twig that diversified into the "true plants." These include mosses (phylum Bryophyta), ferns (phylum Pteridophyta), conifers (phylum Coniferophyta), and the flowering plants (phylum Anthophyta).

Yet another eukaryotic branch lost the chloroplast and evolved into sister groups now known as two separate kingdoms: Fungi and Animalia.

For the purpose of this course, what is important here is that plants have evolved in the context of a tree of evolution from a common ancestor at the trunk. They constitute a large part of a kingdom that share synapomorphies justifying their kingdom as a group.

The Plant Body is Composed of Organs

The vegetative body of a plant consists of three major organs: the leaf, the stem, and the root. These are depicted morphologically (shape and outside form) and anatomically (internal vascularity and cytology).

The shoot is an organ system composed of both stem and attached leaves. The leaves attach to the stem at areas of restricted stem elongation called nodes. Together the blade and petiole constitute a leaf. What is shown here is a simple leaf with one blade at the end of the petiole. Other species may have compound leaves, with more than one blade at the end of the petiole (palmately compound) or along the side of the petiole (pinnately compound). Also attached at the node of the stem is an axillary (lateral) bud. This can differentiate into either a branch shoot or a flower depending upon species and environmental conditions. The most critical function of the leaf is photosynthesis, but also vital is evaporative cooling.

The leaf has an upper and lower epidermis, covered with cutin (wax) to reduce water loss. The upper epidermis has window/lens functions to permit light to enter the leaf. The palisade mesophyll is specialized for photosynthesis. The spongy mesophyll can do some photosynthesis but is a major site of evaporative cooling. The lower epidermis includes guard cells that can regulate the size of the stoma between them and thus regulate gas exchange. Between the two layers of mesophyll are veins. Veins have xylem tissue to conduct water and minerals from the soil, up the plant, and into the leaf for photosynthesis and evaporative cooling. Veins have phloem tissue to conduct sugar and amino acids from the leaf to the rest of the plant.

The stem is composed of the nodes, where leaves and lateral buds are attached and therefore is an area of restricted stem elongation, and internodes, the portions of the stem between nodes that elongate tremendously in some species and in some seasons. In some species there may be a portion of the stem beneath the cotyledons called the hypocotyl. It is in the lowest portion of the stem that the vascular systems transition from that of the root into that of a stem. The critical function of the stem is positioning and supporting the leaves in sunlight, but also vital is transporting water and minerals up from the root and water, sugar, and organic solutes down from the leaves to the root, or up to flowers, fruits, and seeds.

The stem grows from its tip, the apical bud or apical meristem. Here leaf primordia mound up and develop into young leaves, and the nodes and internodes are laid down indefinitely in typical cases for further indeterminate growth. In plants with determinate growth patterns, the apical bud may convert completly over into a flower or inflorescence, halting the vertical growth of the main shoot. After this, it is likely that lateral buds on the stems may grow out into branches that may over-top the main stem.

Below the soil the plant continues in the form of roots. A central stout primary root may develop in girth to form a taproot as in carrots, or it may send out many lateral roots to form a branching fibrous root system. Many plants compromise and have both. These penetrate the soil by growth just proximal to the root tip or root apex. The penetration requires secretion of a lubricant and loss of slough cells from the surface of the root cap. In young areas of root where elongation has ceased, the root may develop almost microscopic root hairs to increase the surface area for water absorption, and to release acid onto soil particles to perform cation exchange, releasing minerals for uptake into the root. Anchorange would be a critical function for the root, but uptake of water and minerals from the soil would be vital.

The Plant Grows from its Apices

From the shoot apical meristem, the plant can add new vertical height to the stem with more nodes and internodes and can create appendages represented by leaf primordia that mature into leaves. The apical meristem is an area with meristematic (rapidly dividing) cells. These add more cells to the apical dome. On the flanks of the shoot apical dome, leaf primordia mound up, elongate, and differentiate into leaves. Between a very old primordium at the edge of the photo and the stem is an axillary bud primoridium, yet another mound of cells from the apex that develops ultimately into an axillary bud.

The root apical meristem grows to add more length to the root with more lateral roots and more root hairs. The root cap proliferates by internal mitoses and cells slough from the surface and the cells secrete mucilage which lubricates the channel through which the growing root penetrates the soil. Behind the padding and frictional insulation offered by the root cap is the actual apical meristem. Again, here the cells divide rapidly to add to the length of the root. Here again, the three embryonic tissues can be easily observed: the protoderm is the surface layer becoming the epidermis, the provascular (aka procambium) cells form a central cylinder that differentiates into xylem and phloem, and the ground meristem in between which becomes the cortex.

Both shoot and root meristems show some early differentiation of the dividing cells into three embryonic tissues: protoderm (a skin-like outer surface layer becoming the epidermis), provascular (aka procambium, elongating cells down the middle of the primordia becoming xylem and phloem), and the ground meristem (the filler cells becoming cortex, pith, and mesophyll).

The Plant Organs are Composed of Tissues

The three mature organs are composed of three fundamental tissues which arise, not surprisingly, from the three embryonic tissues.

The protoderm matures on shoot and root to form the dermal tissues. Generally these are found as a single-cell-thick layer of epidermal cells. However the epidermis may be covered with glands for wax or chemical defense production and secretion or hairs. In leaves and stems, the waxy coating of the cuticle of the epidermis means that a pathway for gas exchange must develop in the epidermis. Stomata have evolved to fill that role. Usually two guard cells surround the stoma and push each other apart with turgor pressure to open the stoma. When the guard cells lose turgor, they collapse together to close the stoma. For the shoot the dermal tissues have window functions, gas exchange functions, and defend against herbivores. The epidermis of roots is not waxed over with cuticle so there are no guard cells. However, root hairs grow outward from epidermal cells in non-elongating zones of the root and these release hydrogen ions to liberate mineral ions from clay particles in the soil. They increase the surface area for water and mineral intake as well.

epidermis cortex pith vascular bundle Stem Cross Section epidermis cortex pith phloem fibers functional phloem vascular cambium xylem

The provascular tissue differentiates into xylem and phloem tissues. Xylem is a complex tissue including cells which lose their cytoplasm at maturity and serve as pipes for conducting water and minerals from the root, through the stem, and up into the leaf. The xylem changes in the transition zone from a single cylinder in the root into a dissected cylinder of vascular bundles in the stem. These bundles branch as needed and connect through the petiole and into the leaf. There the xylem is part of each vein that branches through the mesophyll of the leaf.

The phloem is also a complex tissue with many cell types, but it posesses elongated cells shown best in the leaf cross section which have a cytoplasm of simple design. These living cells conduct the sugar of photosynthesis in the leaf back into the stem and either up to flowers, fruits, seed, or apical bud, or down the stem and into the root. How living cells can conduct bidirectionally will be discussed later. Again the phloem follows the veins, to bundles, to single cylinder form outlined in the xylem.

The ground meristem matures into a range of tissues in various regions. In leaves, the ground meristem forms the palisade and spongy mesophyll which are jointly responsible for photosynthesis in the plant. In the stem the ground meristem regions include the cortex and pith. The cortex has support functions provided by collenchyma and biochemical functions including photosynthesis and chemical defense and storage. The pith is generally considered a water storage region. In the root, the ground meristem differentiates into a nutrient storage cortex, a selective mineral uptake endodermis, and a branch-root forming pericycle.