Plant Biology

Plants’ functional organization is closely linked to their anchored lifestyle and their environment.
They gather food underground and in the sky.

Basic morphology of vascular plants reflects their evolutionary history as anchored terrestrial organisms that inhabit and draw resources from two different environments, below the ground and above the ground. The ability to acquire those resources efficiently relies on important surfaces of exchanges within the root system and the shoot system.

Below the ground surface, roots absorb water and minerals mostly thanks to root hairs. In the atmosphere, stems and leaves – the shoot system – absorb light in chlorophyll cells and carbon dioxide mainly through the leaves stomata. 

Both roots and shoots depend on each other.
Vascular tissue insures material transports within the plant.

The root system is not photosynthetic; it starves unless sugars produced during photosynthesis are imported from the shoot system. Conversely, the shoot system needs the water and minerals that roots get from the soil. The vascular tissue system carries out long-distance transport of materials between the root and shoot systems. The two types of vascular tissues are xylem and phloem. Xylem conducts water and dissolved minerals (xylem sap) upward from the roots into the shoots. Phloem transports the products of photosynthesis (phloem sap), from where they are made – usually the leaves – to where they are needed – usually the roots and growing organs. Water properties, transpiration and hydrostatic pressure gradients insure saps flows.

Evolution has selected many types of plant adaptations against their changing environment.

Roots, shoots and their vascular tissue can adapt regarding the environmental conditions (heat, humidity, minerals concentrations…). Stomata can open or close regarding heat and humidity as well as shoots can grow towards the light thanks to hormones control.

Plants have furthermore developed different anatomical and physiological defense mechanisms linked to their anchored way of life:

• Against environment aggressions = Thick waxy cuticle, stomatal crypts, ability for the leaf to roll in bad conditions…
• Against predators = Thorns, repulsive or toxic secretions…
• Against the seasonal cycle = Bud protection, growth stop, loss of leaves, food storage, dormancy state…

Flowers are complex structures selected by evolution to optimize plants reproduction.

Flowers, the reproductive shoots, are typically composed of four whorls. Entering progressively in the heart of the flower, one can find sepals then petals, stamen (another and filament) and finally carpel (ovary, style and stigma). Sepals and petals are sterile whereas stamen and carpel are reproductive organs (male and female respectively). Development genes control precisely the identity of those four whorls and an ABC hypothesis has been proposed based on plant mutations studies.

Pollination brings male and female gametes close.
It can be achieved thanks to wind or animals.

Fertilization needs the encounter of male and female gametes. In plants, the male gametes are contained in pollen grains within the stamen and ovules are kept in the ovary. Because individuals cannot move and reproduction needs most of the time pollen from a different flower to fertilize the ovules, pollen is disseminated thanks to wind or animals. This step is called pollination. When animals are needed, plants attract them with colors, smells, or nectar. This collaboration, called mutualism, between the animal and the plant comes from a coevolution of both species.

Fertilization produces a seed wrapped in a fruit.
Seed dispersal is possible thanks to gravity, wind, water or animals.

Thanks to pollination, male and female gametes can meet. Pollen grains germinate on the stigma and fertilization can occur. It consists in two fusions: one sperm cell fertilizing the oosphere to form the embryo, the other fertilizing two polar nuclei to form a triploid storing structure called albumen. If fertilization happens, the ovary turns into a fruit and ovules into seeds.

Other floral organs disappear. Species survival and colonization of new territories will be achieved with seeds dispersal. This will involve fruit dispersal thanks to wind, water or animals. Again, in the latter case, coevolution is very important!