Brent Cornell

Plants gain nutrition via the process of photosynthesis, which involves the conversion of light energy into stored chemical energy

• This involves a specialised organelle (chloroplast) to combine inorganic nutrients (CO₂ and H₂O) into organic compounds (glucose)

The tissues of a vascular plant are specifically organised to maximise this process:

• Leaf tissue is organised to optimise light absorption and gas exchange, while root tissue is organised to optimise water uptake
• The vascular system in plants is organised to facilitate the movement of both water (xylem) and nutrients (phloem) between tissues

Leaf Tissue

Leaves possess two main layers of tissue (called mesophyll) that are organised to optimise both light absorption and gas exchange

• Palisade mesophyll is the site of photosynthesis and hence is located on the upper surface of the leaf (facing sunlight)
• Spongy mesophyll is the main site of gas exchange and is hence located on the lower surface of the leaf (near stomata)
• Stomata are on the underside of the leaf (prevents obstruction so as to maintain an open channel for gas exchange)
• The top surface of the leaf is covered by a thick, waxy cuticle (prevents unregulated water loss and protects against pathogens)
• Vascular bundles (a transport system for water and nutrients) are located centrally to allow for optimal access by all leaf cells

Root Tissue

Roots possess three main layers of tissue that are organised to optimise the absorption of water and dissolved minerals from the soil

• The outer epidermis is where water and minerals enter the roots and may have protrusions (root hairs) to maximise surface area
• The middle layer (cortex) contains loosely packed cells that improve aeration and additionally functions as a storage site for starch
• The innermost layer is the vascular tissue (arranged into a central cylinder), which allows for the transport of absorbed materials

Root systems are typically highly branched to maximise material absorption from the soil – the two main types of root systems are:

• Fibrous roots: Thin and very spread out to improve absorption, but not very effective at anchorage (more suitable for smaller plants)
• Tap roots: A deeply penetrating main root with many lateral branches (can access deeper soil layers and provides better anchorage)

Vascular Tissue

Material is transported between the leaves and the roots via the vascular tissue within the stems of the plant

• Water is absorbed via the roots and is transported to the leaves via a vascular vessel called the xylem
• Organic molecules are produced by photosynthesis in the leaves and are transported via a vessel called the phloem 

Xylem vessels form a hollow tube composed of an outer layer of dead cells that are pitted with gaps to facilitate water exchange

• The xylem vessel is subject to high hydrostatic pressures and so is reinforced with lignin (in a spiral or annular arrangement)

Phloem vessels are composed of sieve element cells that are connected by porous sieve plates to form a continuous tube

• Phloem vessels are supported by companion cells, which provide metabolic support and facilitate the loading of nutrients