Introduction to Plants

• Plants are multicellular, predominantly photosynthetic eukaryotes in the kingdom Plantae.
• Plants form a clade that includes the glaucophytes, red algae, green algae, and land plants.
• There are more than 300,000 species of extant plants.
• Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria.
• Plants are vital to life on Earth because they generate most of the oxygen and food that humans and other organisms breathe and eat.
• Plants called embryophytes, which include the land plants (seed plants, ferns, lycophytes, and mosses), evolved from specialized green algae.
• Land plants adapted to terrestrial environments and are characterized by the presence of an embryo and specialized tissues.
• The scientific study of plants is known as botany.

Vascular Plants

• Vascular plants, also known as tracheophytes, possess specialized vascular tissues.
• These tissues, xylem and phloem, facilitate the transport of water, minerals, and photosynthetic products throughout the plant.
• Vascular plants include the majority of familiar plant species, such as trees, shrubs, grasses, and flowering plants.
• They are characterized by true roots, stems, and leaves, which are interconnected by the vascular system.
• The evolution of vascular tissue allowed plants to grow taller and colonize a wider range of terrestrial habitats.
• Vascular plants represent about 90% of the extant plant species

Non-Vascular Plants

• Non-vascular plants, also known as bryophytes, lack specialized vascular tissues.
• They include mosses, hornworts, and liverworts.
• Non-vascular plants are typically small in size, often forming mats or cushions on moist surfaces.
• They lack true roots, stems, and leaves.
• These plants rely on diffusion and osmosis to transport water and nutrients, limiting their size and distribution.
• Bryophytes are commonly found in moist habitats, such as forests, wetlands, and stream banks.
• They play important ecological roles, such as soil stabilization, nutrient cycling, and habitat provision for various organisms.
• They require moist environments for reproduction, as their sperm cells must swim to reach the eggs.

Xylem

• Xylem is a complex vascular tissue responsible for transporting water and minerals from the roots to the rest of the plant.
• It is composed of specialized cells called tracheids and vessel elements, which are dead and hollow at maturity.
• The cell walls of xylem cells are reinforced with lignin, providing structural support and preventing collapse under negative pressure.
• Xylem transport is driven by transpiration, the evaporation of water from the leaves, which creates a tension gradient that pulls water upward through the xylem.
• Xylem also provides structural support to the plant stem and branches, allowing them to withstand wind and other environmental stresses.

Phloem

• Phloem is another complex vascular tissue responsible for transporting sugars, amino acids, and other organic compounds from the leaves to other parts of the plant.
• It is composed of specialized cells called sieve tube elements and companion cells.
• Sieve tube elements are living cells that are connected end-to-end to form long sieve tubes.
• Companion cells provide metabolic support to the sieve tube elements and regulate phloem transport.
• Phloem transport is driven by pressure flow, where sugars are actively loaded into the phloem at the source (e.g., leaves) and unloaded at the sink (e.g., roots, fruits).
• Phloem transport is bidirectional, meaning that it can transport substances in both directions, depending on the needs of the plant.

Roots

• Roots are underground organs that anchor the plant to the soil and absorb water and nutrients.
• Vascular plants have true roots that are specialized for absorption and anchorage.
• Non-vascular plants lack true roots but may have root-like structures called rhizoids that help anchor the plant.
• Roots have a large surface area for absorption, due to the presence of root hairs, which are extensions of epidermal cells.
• Roots also interact with soil microbes, such as mycorrhizal fungi and nitrogen-fixing bacteria, which enhance nutrient uptake.
• Roots can also store carbohydrates and other nutrients, providing a reserve for the plant during times of stress.

Stems

• Stems are aboveground organs that support the leaves and reproductive structures of the plant.
• Vascular plants have true stems that contain vascular tissue (xylem and phloem) for transport.
• Non-vascular plants lack true stems and have simpler structures called stalks or stolons.
• Stems provide structural support to the plant, allowing it to grow upright and reach sunlight.
• Stems also transport water, nutrients, and sugars between the roots and the leaves.
• Stems can also store carbohydrates and other nutrients, providing a reserve for the plant during times of stress.

Leaves

• Leaves are the primary photosynthetic organs of the plant, responsible for capturing sunlight and converting it into chemical energy.
• Vascular plants have true leaves that are typically flat and broad, maximizing surface area for light absorption.
• Non-vascular plants lack true leaves and have simpler structures called phyllids or thalli.
• Leaves contain chloroplasts, which are organelles that contain chlorophyll, the pigment responsible for capturing light energy.
• Leaves also have stomata, which are pores on the leaf surface that allow for gas exchange (uptake of carbon dioxide and release of oxygen).
• Leaves can also be modified for other functions, such as storage, protection, or attraction of pollinators.

Reproduction in Vascular Plants

• Vascular plants have complex reproductive strategies that involve both sexual and asexual reproduction.
• Sexual reproduction involves the fusion of male and female gametes (sperm and egg) to produce offspring with genetic variation.
• Vascular plants have evolved various mechanisms to ensure successful pollination and fertilization, such as flowers, fruits, and seeds.
• Asexual reproduction involves the production of offspring from a single parent, without the fusion of gametes, resulting in genetically identical offspring.
• Vascular plants can reproduce asexually through various methods, such as vegetative propagation, fragmentation, and apomixis.

Reproduction in Non-Vascular Plants

• Non-vascular plants have simpler reproductive strategies that rely on water for fertilization.
• Sexual reproduction in bryophytes involves the fusion of motile sperm cells with non-motile egg cells within specialized structures called archegonia.
• Bryophytes require moist environments for reproduction, as the sperm cells must swim to reach the eggs.
• Non-vascular plants can also reproduce asexually through fragmentation or the production of gemmae, which are small, detachable structures that can grow into new plants.
• Non-vascular plants have a life cycle that alternates between a haploid gametophyte generation and a diploid sporophyte generation.
• The gametophyte is the dominant generation in bryophytes, while the sporophyte is dependent on the gametophyte for nutrition.