. The Invasion of the Land is really the Invasion of the Atmosphere!!!
The Protoplasm of Individual Plant Cells is surrounded by a Cellulose Wall. While Cellulose is strong and prevents mechanical damage to the cell contents, it is extremely hydrophilic and readily absorbs water.However, Cellulose easily loses water via evaporation.
. Cellulose is like a sponge. If you drop a sponge in water, it saturates instantaneously. A wet sponge readily loses water when it is placed on a dry substrate. In order for an isolated plant cell, like a unicellular alga, to survive, it must be in constant contact with water.
In order to withstand periodic dry spells, plant cells needed a water protective coating.
. One of the most important plant adaptations is the Cuticle. It is a waxy material that is secreted to the outside of the plasma membrane. It fills in the spaces between cellulose fibrils and forms a continuous external waxy layer to the outside of the cell wall. This makes the cell watertight!
. This cell can be called an "all purpose" cell because it Regulates its water balance and performs Photosynthesis.
The Cuticle keeps water inside but it also prevents water uptake. The Cuticle is usually thicker on the side of the cell facing the light. Consequently, water could enter the bottom of the cell where the cuticle is thin and where water is more
. abundant, and be retained within the cell by the thick cuticle on its upper side. This could lead to the formation of colonies. The first multicellular forms could be filaments. These might be followed by flat sheets.
The Chlorophyta (Green Algae) is algal group which probably gave rise to land plants. The genus Coleochaete is regarded as the
. closest relative for early land dwellers. It is a disk-like organism that is several cells thick in the center and one cell thick at its margins. It has marginal growth rather than apical growth This means that cells along the perimeter of he disk are mitotic and growth spreads around the edge of the organism.
. Advantages of Multicellularity
Increased Volume of the Organism greatly increases the volume and total surface area of the cells and their membranes.
This enhances the ability to exchange chemicals with the environment, and provides local reservoirs for essential nutrients.
. It allows the separation of absorptive and non-absorptive regions of the organism. This leads to cell and tissue specialization.
The accumulated effect of the cellular exoskeleton (cell walls) and the turgor pressure of many cells combined, provides a greater degree of structural support. This protects the organism from physical forces like the movements of air and water.
This also insulates or buffers them from minor, local changes in temperature or moisture as well as nutrient availability.
This also leads to larger organisms which can "out compete" other organisms for the resources available in the environment.
Parts of the plant may survive attack by an herbivore or pathogen and can regenerate complete organisms.
. Larger organisms tend to have longer life spans.
Multicellularity also lends protection to reproductive structures.
It is inefficient for one cell to perform a multitude of functions. Consequently, cell specialization evolved.
. The most important basic functions for the survival of land plants are
performing photosynthesis and
transporting photosynthate (sucrose dissolved in water).
All of these involve water!
(Hepatophyta & Anthocerophyta)
Vegetative Morphologies similar to those above are seen in the Thallose Liverworts (Hepatophyta) and Hornworts (Anthocerophyta). Some of these have wide, flat spreading thalli which are translucent and chlorophyllous throughout. They may be only 1-2 cells thick at their margins.
. Thallus means a sheet-like form which does not have any resemblance to typical plant organs like leaves, stems or roots. The thallose liverworts have thalli which resemble green algae. In some cases all of the cells contain chloroplasts and the marginal areas are only one cell thick.
. Some display Marginal Growth like Coleochaete and have Chloroplasts that are similar to the Green Algae (Chlorophyta).
. The next level of complexity which arose could have been a multilayered Thallus.
The cells on the bottom layer could have been specialized for water absorption. They might have had a thin Cuticle and hair-like projections called Rhizoids. Rhizoids are like root hairs. They are specialized for the absorption of water.
. They also anchor the thallus to the substrate. This would constitute a Tissue called Epidermis.
The next step could be the formation of a separate photosynthetic layer composed of Photosynthetic Tissue called Chlorenchyma (a.k.a. Photosynthetic Parenchyma).
. The upper layer would loose its photosynthetic abilities and become more specialized for water retention. It would constitute another layer of Epidermal Tissue. The Chlorenchyma is thus embedded within the two specialized Epidermal layers.
The next major advance in plant evolution was the Stomata or a stomata-like pore! This is one of the most important plant adaptations.
. It allows plants to regulate gas exchange with the atmosphere.
A solid Epidermis with a thick cuticle would hamper gas exchange because Oxygen and Carbon Dioxide can not readily pass through the Cuticle. Photosynthetic cells need atmospheric Carbon Dioxide which is present in very low concentrations. Consequently, holes in the Epidermis would allow more Carbon Dioxide to
. enter the thallus. However, water vapor would leave simultaneously at a high rate. This would desiccate and damage the internal tissues.
However, physiological control over the opening and closing of the stomata allows the plant to balance these two conflicting processes (CO2 uptake and H2O loss).
The formation of air spaces in the Chlorenchyma accompanied the
. formation of stomata. Such tissue is called Aerenchyma. These internal air spaces create gas reservoirs where Carbon Dioxide can accumulate so that photosynthesis can proceed when the stomata are closed. This also insures an even distribution of gases within the photosynthetic tissue.
. Further specialization might have led to the formation of many layers and different shapes of Chlorenchyma to maximize photosynthetic capacity.
Our theoretical Organism has developed TISSUES. Tissues are groups of cells which are structurally/functionally distinct.
The Genus Marchantia (Hepatophyta) displays many of these traits.
. These sections through a Marchantia Thallus display the level of structural complexity achieved by this plant. We have already noted features of the Dorsal and Ventral Epidermis. Layers of Chlorenchyma is found directly below the Dorsal Epidermis. The photosynthetic cells are slightly columnar and there is a lot of air space between the cells. This is similar to the Mesophyll found in some "advanced" land plants.
. Note the way in which the Chlorenchyma distributed in relation to the Epidermal Pores. This is very precise and probably represents the most efficient distribution of Photosynthetic Cells and Pores. It would be interseting to look a Marchantia grown in different environments to see if this relationship varies with the environment. A layer of storage Parenchyma subtends the
. Chlorenchyma. The Ventral Epiderms produces Rhizoids which anchor the thallus and absorb water. Consequently, we see distinct layers which have different functions with regards to Photosynthesis and Water relations.
. In order for a complex colony or organism to survive, water and sucrose must be distributed throughout the organism. Cells are interconnected by small cytoplasmic channels in their cell walls called Plasmodesmata. Furthermore, small molecules can move by diffusion through the water in the Cell Walls. However, both of these process are extremely slow and could not be
. sufficient for large organisms to survive.
Vascular Tissues are specialized for the transport of water and sucrose dissolved in water. The most simple Vascular plants have one vascular bundle composed of Xylem and Phloem.
Xylem is specialized for Water Transport BUT also supplies Structural Support!
. This is required for Vertical Growth.
Phloem is specialized for sucrose transport.
Some Thallose Liverworts have a central Nerve (Midrib) that contains cells modified for conduction but they do not approach the levels of differentiation seen with Xylem and Phloem. In the most advanced organisms of this type, highly
. elongated cells with some wall thickening occupy the center of the Nerve.These resemble Tracheids and are called Hydroids. However, they lack most of the features associated with Tracheary Elements in seed plants.
. Leafy Liverworts generally live in wet environments like those on the right, above. They can be important pioneer organisms which help to stabilize soil. They clamber about on the substrate and do not produce markedly vertical leafy stems.They have few adaptations that protect them from desiccation BUT they have an amazing capacity to tolerate
. desiccation without losing vitality and they can be revived when water becomes available. This picture on the left above was taken along the Manoa Cliffs Trail in an area that was exposed to sunlight.
. Mosses (Bryophyta)
The next step in Evolution was a quest to become Vertical. Plants that could grow above Thalloid organisms would have a distinct adaptive advantage as they could intercept light and shade out their competitors. This requires the production of strengthening tissues and conducting tissues.
. The Mosses (Bryophyta) are the first group to develop leafy, vertical shoots. They use the same strengthening and conducting tissues that were found in the Liverworts but they produce them in sufficient quantities to achieve verticality. These are still small plants which rarely top 10 cm.
. The most complex species had photosynthetic and subterranean stems. The latter are root-like and produce hairs called Rhizoids.
These are still Small Plants reaching
few cm. in height but they tower over the Hepatophyta & Anthocerophyta.
. Many species are terrestrial but a significant number are epiphytes. A few are aquatic or semi-aquatic.
They typically grow in wet areas but some can grow in extremely cold and dry environments where they are Pioneers.
They can have minute Leaves but some species have comparatively complex Leaves which have a Nerve that has conducting & strengthening tissues.
Hydroids do NOT have the type of Secondary Cell Wall Thickenings which are typical for Tracheary Elements of other land plants.
The Equisetum Tracheid (left) has the kind of Secondary Walls that are typical for Tracheids in general.
The Moss Hydroid (right) lacks the Secondary Wall thickenings. However it has the same overall shape of a tracheid.
. Thallus Shape
Thallus Shape is important because plants need to minimize water evaporation. A sphere has the least amount of Surface Area / Volume ratio.
A flat sheet has the greatest Surface Area / Volume ratio.
Water would evaporate more readily from the sheet because more
. molecules are in direct contact with the atmosphere.
However, plants need to perform Photosynthesis in order to survive. Consequently, plants need to maximize their surface area to intercept as much light as possible. The process of evolution has selected plants which are best able to balance these two conflicting needs.
. The first land plants probably resembled a discus which was thin at the margins and thick in the middle. This is similar to certain Green Algae (Coleochaete). However, this shape has a relatively Large Surface / Volume ratio.
Some Thallose Liverworts (Anthoceros) have a shape like this but there are no large land plants with this kind of shape. Consequently, other shapes must have had greater adaptive value.
. A strap-like thallus would have less surface area compared to a disk-like thallus and there are some land plants like Marchantia that have this kind of shape. This kind of thallus would be multilayered in its center and unilayered at its margin.
A Cylinder has even less Surface Area / Volume than a strap-like structure. The first fossil land plants had cylindrical thalli and
. some extant plants have retained this shape. These thalli would be similar to stems and roots.
The first cylindrical organisms probably had horizontal photosynthetic stems called Stolons.
underground stems called Rhizomes developed later and eventually upright, aerial, photosynthetic stems arose.
. All of these had a simple Tissue organization of with Vascular Tissues in the center, surrounded by Ground Tissue and Epidermis.
Initially, all of these Organs had one central strand of Vascular Tissues (Xylem & Phloem), a cylinder of Photosynthetic Parenchyma (Chlorenchyma) and an Epidermis.
. The Psilophyta contains a genus called Psilotum which closely resembles plants like Rhynia. It is essentially a stem that has two interconnected forms.
Its Underground Stem (Rhizome) produces Aerial Stems which are Photosynthetic..
The Leaves are minute and contribute little to the plant's nutrition.
The Rhizome produces Rhizoids which act like root hairs
. Psilotum Underground Rhizome: Some Rhizome tips start to grow upwards and are converted into photosynthetic shoots.
No Roots are produced!!
The Rhizomes serve as roots and they produce Rhizoids which act like root hairs.
. The Xylem of Psilotum contains Tracheids which are similar to those seen in other Vascular Plants. These have characteristic Secondary Wall Thickenings, unlike the Hydroids we examined earlier.
The Phloem contains Sieve Elements which are similar to those seen with other Vascular Plants, unlike Leptoids which do not have all the features of Sieve Elements.
. The anatomy of the Aerial Stem is similar to that of the Rhizome. However, the Xylem is star-shaped rather than circular and the Ground Tissue contains Sclerenchyma and Chlorenchyma.
. There is a debate concerning the
Leaves of Psilotum. They do not have Veins although Leaf Traces may diverge from the Stele and approach the leaf bases. I think that they are leaves. If we assume that they had one Vein, they would be Microphylls.
The next step lead to the evolution of distinct Shoots and Roots.
. The Shoots are Aerial structures specialized for Photosynthesis while the Roots are subterranean structures specialized for Absorption and Anchorage. Which came first?
Root anatomy would not differ significantly from the cylindrical Rhizome we saw earlier. Root Anatomy is very constant in Land Plants.
. Branching eventually evolved in roots. This produced a complex root system which can be just as extensive as the aerial system. The root system may be more extensive that the shoot system in dry (Xeric) environments.
Upright stems were able to overtop flat thalli and became the dominant organisms.
. The production of photosynthetic branches further increased the surface area available for photosynthesis. Branching was initially Dichotomous for stems and roots. Lateral branching developed later in Evolution.
The ascent of aerial stems and their branches required the development of superior support tissues. This was achieved with the Psilophyta.
. Xylem Tracheary Elements have thick walls and provide structural support. However, Sclerenchyma (Scler = Hard) tissue evolved and Sclerenchyma Fibers provided extra mechanical strength. Fibers are found in the Vascular Tissues or in close association with them. Sclerenchyma and Parenchyma are the two principal Ground Tissues. Xylem & Phloem can contain both of the above.
. The final step in the evolution of
our theoretical plant is the production of Leaves. These are highly specialized
for Photosynthesis in many ways.
They usually have a wide thin Blade (Lamina) and a thicker Midrib. They may have only one Vascular Bundle (Vein) or a network of interconnected Veins that differ in diameter.
. They are attached to the stem by a nonphotosynthetic petiole.
The first Leaves were Microphylls and had one Vein/Leaf. The vein had a central location and was surrounded by Photosynthetic Parenchyma. An Epidermis with Stomata covered the entire Microphyll.
. The major functions of the stem are translocation of water and photosynthate, and structural support for the leaves & branches.
Our theoretical plant has three distinct Organs (Leaf, Stem & Root).
Species in the Lycophyta illustrate simple plants that have Microphylls.
Plants in the Lycophyta have erect stems as well as Stolons and Rhizomes. They are relatively large compared to Hepatophyta, and Bryophyta but they rarely exceed a meter in height. They can be epiphytes and their pendant stems can be more than a meter in length.
They have Microphylls and Roots.
Branching is Isotomous Dichotomous for both organs. Isotomous means that each branch develops equally.
The Apical Meristem can have several "Initials" rather than a solitary Apical Cell. This is somewhere between an Apical Cell and the Multicellular Apices of flowering plants.
We will examine the genus Lycopodium first.
. The Stolons also produce Aerial Shoots which have Anisotomous (Unequal) branching. When dichotomous branches develop unequally (on=short & 1=long) the branching is called Anisotomous. The upright stems are about 1 m in height.
The species to the right grows locally on disturbed sites and could . be a candidate for soil stabilization
research. It is a complex plant which has horizontal Stolons which have Isotomous Branching. These produce the Roots which anchor the plant to the substrate. The Stolons also produce Aerial Shoots which have Anisotomous (Unequal) branching. When dichotomous branches develop unequally (on=short & 1=long) the branching is called Anisotomous. The upright stems are about 1 m in height.
. While extant Lycopods are small plants with little ecological significance. Forests of tree-sized lycopods once dominated certain habitats. The most famous of these is Lepidodendron which reached heights up to 30 meters. They had secondary growth. The stems were coated with leaf bases and there appeared to be little internodal elongation.