The structure of the leaf Flashcards
Give 2 words for the study of the external structure of plant organs
morphology/organography
Where are leaves typically found
In vascular plants, which have lignified tissues (xylem) that enable them to conduct water
Why are leaves usually flat and thin
To allow for maximum gas exchange and capture of light.
The organisation of the leaf has evolved to allow maximum exposure of chloroplasts to light and to absorb carbon dioxide
What do the stomata in leaves do
They allow the plant to regulate the exchange of carbon dioxide, oxygen and water vapour with the atmosphere
Describe the two types of leaves
- Dorsiventral
have both surfaces differing from each other in appearance and structure - Isobilateral
have both surfaces looking the same
What is a leaf
A flattened organ with limited growth, born at the node of a stem and develops form a leaf primordium just below the apical meristem of the growth point of the stem
Describe the typical structure of a leaf
- Green and contains lamina (leaf blade), a petiole(leaf stalk) and a leaf base where it is attached to the stem.
Leaf base cant always be clearly distinguished . In grass the leaf base forms a sheath that surrounds the stem - Petiole attaches the lamina to the stem
It occurs in petiolated leaves only (a sessile leaf does not have a petiole)
The petiole mainly consists of vascular tissue that runs from the stem to the leaf blade
What is the venation of a leaf
The arrangement of the vascular tissue in the leaf
What is the mid rib
Main/primary vein that is the most superficial vein. Lateral veins (secondary veins) branch from the mid rib and usually branch further, forming tertiary veins
Leaves are grouped into leaves with a parallel venation (mostly monocotyledons) and pinnate or palmate venation (dicotyledons)
What forms reticulate patterns in leaves
The smallest veins in mono and dicotyledons
Describe the division of the lamina
- Undivided in simple leaves, divided in compound leaves
- When the lamina of a single leaf is subdivided into smaller units up to the midrib or other prominent veins, the leaf is now compound and the smaller units are called pinnae
- The vein bearing the pinnae is called the rachis
- If the pinnae are not divided any further and are born on either side of the rachis, the leaf is pinnately compound
What is the function of leaves
Photosynthesis, transpiration and the transport of organic nutrients produced in photosynthesis
What is transpiration
The release of water in the form of water vapour through the stomata of the leaves
What is photosynthesis
The process whereby sugar (mainly glucose) is produced in the leaves (chloroplasts) from water and carbon dioxide. Sunlight is needed for the process and oxygen is released
What does the lamina of the leaf consist of
The upper (adaxial) and lower (abaxial) epidermis with the mesophyll between them and vascular bundles embedded in the mesophyll. The upper and lower epidermis are covered by a thick, waxy cuticle that covers the entire lamina, except the parts above the stomata
What are the characteristics of the epidermis in leaves
- A single layer of brick shaped cells without intercellular spaces
- Consists of living cells usually without chloroplasts
- Irregular shape in surface view
- Stomata usually occur on the abaxial(lower) side. the function of stomata is to enable the release of water vapour and the intake of carbon dioxide
- Epidermal cells may be modified as living/non living hairs (trichomes)
What are the functions of epidermis in leaves
- Protection against water loss via stomata and a waxy cuticle
- Regulation of gaseous exchange
- Secretion of metabolic compounds
What does mesos and phyllo mean in the word mesophyll
mesos- middle
phyllo- leaf
What is another word for mesophyll
The cortex/ chlorenchyma (because it contains many chloroplasts)
What is the mesophyll
Parenchyma cells with large vacuoles that hold water for photosynthesis
It is the part of the leaf between the upper and lower epidermis, excluding the vascular bundles (veins)
It consists of palisade parenchyma and spongy parenchyma
Describe the palisade parenchyma
- Consists of elongated parenchyma cells that are arranged perpendicular to the adaxial (upper) epidermis
- Can be more than one cell layer and contains numerous chloroplasts
- There are small intercellular spaces, except above stomata, where there are big sub stomatal cavities (air spaces)
- Densely packed to allow glucose to be transported among the cells to the phloem with ease, from there it is transported to the rest of the plant
Describe the spongy parenchyma
- Loosely packed, irregular cells with big intercellular spaces, contains chloroplasts
- Stretches from the palisade parenchyma to the abaxial(lower) epidermis
- The tissue absorbs carbon dioxide(stomata) from the atmosphere (lower and cooler side of the leaf) and transports it to the palisade parenchyma, as well as organic compounds from the palisade to the phloem and water and inorganic compounds from the xylem to the palisade
- Sub stomatal cavities enhance the exchange of gases and water vapour between the tissue and the atmosphere
What does the bundle sheath do
Forms the border between the vascular bundle and the remainder of the mesophyll. Strengthening tissue (collenchyma or sclerenchyma) occurs above and below the vascular bundles
What is the main vein/ mid rib
Represents the primary vascular tissue of the leaf. The xylem lies on the upper side of the circular vascular bundle, with the phloem below. The cambium is present between the xylem and phloem
Describe the stomata in the leaf
Consists of two bean shaped guard cells with a pore and an air chamber (stomatal cavity)
Carbon dioxide enters through stomata (on the lower/cooler side of the leaf)
Stomata open during the day to release water vapour from the xylem, through loosely packed spongy parenchyma cells to the atmosphere.
A pulling force/ continuous column of water develops in the vascular tissues to the roots to enable the absorption of water
Describe the guard cells in the leaf
Contain chloroplasts and starch granules. Irregular thickenings in the walls regulate the size of the pore and thus regulate gas exchange
Describe the pores in the leaf
Connects the outer atmosphere with the sub stomatal cavity and with intercellular air channels
What does the waxy cuticle on leaves do
Limits water loss from the leaf
Why are epidermal cells transparent
so light can penetrate to the deeper layers with large numbers of chloroplasts
What is the function of the network of vascular bundles throughout the leaf
water from the roots can reach all the photosynthesising tissues, and the product of photosynthesis can be taken away
Why is the palisade parenchyma on the adaxial side of the leaf
maximum light absorption
Why are palisade parenchyma cells elongated
deeper penetration of sunlight
Why are palisade parenchyma cells densely packed
They are densely packed with few intercellular spaces to facilitate the diffusion of substances
Why are spongy parenchyma cells loosely packed
makes gas exchange easier
