The structure of the leaf

Question 1

Give 2 words for the study of the external structure of plant organs

Answer 1

morphology/organography

Question 2

Where are leaves typically found

Answer 2

In vascular plants, which have lignified tissues (xylem) that enable them to conduct water

Question 3

Why are leaves usually flat and thin

Answer 3

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

Question 4

What do the stomata in leaves do

Answer 4

They allow the plant to regulate the exchange of carbon dioxide, oxygen and water vapour with the atmosphere

Question 5

Describe the two types of leaves

Answer 5

1. Dorsiventral
   have both surfaces differing from each other in appearance and structure
2. Isobilateral
   have both surfaces looking the same

Question 6

What is a leaf

Answer 6

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

Question 7

Describe the typical structure of a leaf

Answer 7

1. 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
2. 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

Question 8

What is the venation of a leaf

Answer 8

The arrangement of the vascular tissue in the leaf

Question 9

What is the mid rib

Answer 9

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)

Question 10

What forms reticulate patterns in leaves

Answer 10

The smallest veins in mono and dicotyledons

Question 11

Describe the division of the lamina

Answer 11

1. Undivided in simple leaves, divided in compound leaves
2. 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
3. The vein bearing the pinnae is called the rachis
4. If the pinnae are not divided any further and are born on either side of the rachis, the leaf is pinnately compound

Question 12

What is the function of leaves

Answer 12

Photosynthesis, transpiration and the transport of organic nutrients produced in photosynthesis

Question 13

What is transpiration

Answer 13

The release of water in the form of water vapour through the stomata of the leaves

Question 14

What is photosynthesis

Answer 14

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

Question 15

What does the lamina of the leaf consist of

Answer 15

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

Question 16

What are the characteristics of the epidermis in leaves

Answer 16

1. A single layer of brick shaped cells without intercellular spaces
2. Consists of living cells usually without chloroplasts
3. Irregular shape in surface view
4. 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
5. Epidermal cells may be modified as living/non living hairs (trichomes)

Question 17

What are the functions of epidermis in leaves

Answer 17

1. Protection against water loss via stomata and a waxy cuticle
2. Regulation of gaseous exchange
3. Secretion of metabolic compounds

Question 18

What does mesos and phyllo mean in the word mesophyll

Answer 18

mesos- middle

phyllo- leaf

Question 19

What is another word for mesophyll

Answer 19

The cortex/ chlorenchyma (because it contains many chloroplasts)

Question 20

What is the mesophyll

Answer 20

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

Question 21

Describe the palisade parenchyma

Answer 21

1. Consists of elongated parenchyma cells that are arranged perpendicular to the adaxial (upper) epidermis
2. Can be more than one cell layer and contains numerous chloroplasts
3. There are small intercellular spaces, except above stomata, where there are big sub stomatal cavities (air spaces)
4. 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

Question 22

Describe the spongy parenchyma

Answer 22

1. Loosely packed, irregular cells with big intercellular spaces, contains chloroplasts
2. Stretches from the palisade parenchyma to the abaxial(lower) epidermis
3. 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
4. Sub stomatal cavities enhance the exchange of gases and water vapour between the tissue and the atmosphere

Question 23

What does the bundle sheath do

Answer 23

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

Question 24

What is the main vein/ mid rib

Answer 24

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

Question 25

Describe the stomata in the leaf

Answer 25

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

Question 26

Describe the guard cells in the leaf

Answer 26

Contain chloroplasts and starch granules. Irregular thickenings in the walls regulate the size of the pore and thus regulate gas exchange

Question 27

Describe the pores in the leaf

Answer 27

Connects the outer atmosphere with the sub stomatal cavity and with intercellular air channels

Question 28

What does the waxy cuticle on leaves do

Answer 28

Limits water loss from the leaf

Question 29

Why are epidermal cells transparent

Answer 29

so light can penetrate to the deeper layers with large numbers of chloroplasts

Question 30

What is the function of the network of vascular bundles throughout the leaf

Answer 30

water from the roots can reach all the photosynthesising tissues, and the product of photosynthesis can be taken away

Question 31

Why is the palisade parenchyma on the adaxial side of the leaf

Answer 31

maximum light absorption

Question 32

Why are palisade parenchyma cells elongated

Answer 32

deeper penetration of sunlight

Question 33

Why are palisade parenchyma cells densely packed

Answer 33

They are densely packed with few intercellular spaces to facilitate the diffusion of substances

Question 34

Why are spongy parenchyma cells loosely packed

Answer 34

makes gas exchange easier