RHS Level 2 Certificate

Year 1 Week 5: Stems and Leaves

Learning objectives

1. Stems – structure and purpose1.1 Describe how the stem develops.1.2 Describe different types of buds and state where they occur in the plant.1.3 Identify the two elements of the vascular system and name their purposes1.4 Describe transverse sections of the young dicotyledon stem, should

include the location of the following: epidermis, cortex, xylem, phloem, pith, cambium, vascular bundles, endodermis.

1.5 Describe how the stem is adapted in order to perform specific functions.2. Leaves – structure and purpose2.1 Describe the structure of the leaf.2.2 Describe how leaf shape, size, form and colour can vary.2.3 Describe how leaves are adapted in order to perform certain functions2.4 Describe transverse sections of the young dicotyledon leaf; this should

include the location of the following: epidermis, xylem, phloem, vascular bundles, palisade & spongy mesophyll, cuticle, stomata.

Stems – Structure and Purpose

o What are stems for?o support for leaves and flowers – competition for

light and access for pollinatorso transport of water and nutrients around the plant

o What is the internal structure of the stem?o Stem vascular systemo Differences between stem and rootso Differences between monocot and dicot stems.

Stems – vascular system

Vascular system in a number of bundles – unlike the root where there is only one in the centre.

Monocots – scattered irregularly Dicots – arranged regularly around the

circumference Xylem – located on the ‘inside’ of the bundle Phloem – on the ‘outside’ Vascular cambium – meristematic tissue that

generates new xylem and phloem.

Young Dicot stem

Buds – different types

Apical bud – at the tip of the shoot. What is apical dominance? How can this be manipulated by gardeners?

Axillary buds – at the joint between leaf and stem (at the node). Usually dormant or leaf producing.

Stem Adaptations

Water storage – e.g. Opuntia sp. Thorns for protection - Crataegeus

monogyna (Common Hawthorn) Dormancy – storing food to allow avoidance

of hostile conditions over winter or during summer drought e.g. Zingiber officinale (Ginger ) has a rhizome, Solanum tuberosum ssp. Tuberosum (potato) has stem tubers

Leaves –function and structure (1)

Function of leaves – photosynthesis

External structure – Petiole– Lamina– Midrib– Veins

Leaves –function and structure (2)

Cuticle –waxy layer on surface that slows water loss

Epidermis – cells that form ‘skin’ of leaf

Palisade cells – arranged like picket fence, contain chloroplasts

Chloroplasts – site of photosynthesis

Spongy mesophyll – packing cells, allow water vapour and other gasses through

Stomata – active control of entry and exit of gasses and water vapour.

Phloem – dissolved sugars Xylem – water and mineral nutrients

Leaf – internal structure

Stomata

Pairs of specialised guard cells on either side of small openings in the underside of leaves and in soft stems.

Guard cells swell when the plant has enough water – pulling apart to open the stomata and allowing water vapour and waste gasses out and oxygen and CO2 in.

When water levels fall the guard cells empty and the stomata close to conserve water.

Electron-micrograph of lettuce stomata

Leaf Adaptations

Tendrils – for climbing e.g. Lathyrus odoratus Leaf Hairs – for protection from the sun e.g.

Stachys byzantina Spines – for protection from grazing animals

e.g. Ilex aquifolia Food storage – for over-wintering, e.g.

Narcissus spp.

Leaf adaptations (2)

Leaf fall for deciduous trees – recovery of chemical components of chlorophyll; formation of abscission layer, a layer of weak cells; vascular bundles plugged; leaf fall.

Juvenile trees retain dead leaves, protects buds

Leaf form and leaf arrangement (phyllotaxy)

Various different leaf forms – known by different descriptive names to enable clear identification

Phyllotaxy is the name for the arrangement of leaves on a plant. Prevents the leaves from shading each other.

Learning outcomes

1. Stems – structure and purpose1.1 Describe how the stem develops.1.2 Describe different types of buds and state where they occur in the plant.1.3 Identify the two elements of the vascular system and name their purposes1.4 Describe transverse sections of the young dicotyledon stem, should include the

location of the following: epidermis, cortex, xylem, phloem, pith, cambium, vascular bundles, endodermis.

1.5 Describe how the stem is adapted in order to perform specific functions.2. Leaves – structure and purpose2.1 Describe the structure of the leaf.2.2 Describe how leaf shape, size, form and colour can vary.2.3 Describe how leaves are adapted in order to perform certain functions2.4 Describe transverse sections of the young dicotyledon leaf; this should include

the location of the following: epidermis, xylem, phloem, vascular bundles, palisade & spongy mesophyll, cuticle, stomata.