Lecture 3 Roots and Stems Zivuku. M, 2015 Lecture 3 Roots, Stems, Leaves

STRUCTURE OF MODIFIED ROOTS AND STEMS AIM – be able to: Differentiate between monocots and dicots roots Discuss and sketch the external and internal structure of roots and stems , Longitudinal and cross sections through both monocotyledonous and dicotyledonous roots Describe the function and give examples of different kinds of modified roots and modified stems.

Dicotyledonous roots Xylem “arms” X to conduct water and dissolved nutrient minerals Phloem between “arms” to transport dissolved sugar (sucrose) Vascular cambium between xylem + phloem

How do Monocotyledonous roots differ ? Monocotyledonous roots have a central pith Different arrangement of vascular tissue –Xylem in a ring No vascular cambium

Dicotyledonous vs Monocotyledonous

Modified roots – Food storage Tap roots – Carrots, beetroot Lateral roots - Tubers Sweet potatoes

Modified roots – Aerial Prop roots Prop roots – Adventitious roots from base of stem to give extra support e.g. Mielies, Zea mays

Modified roots – Aerial Buttress roots Shallow roots in moist forests to absorb dissolved nutrient minerals from decomposing leaves Buttress roots support shallowly rooted trees in damp soil and can cover large areas e.g. Ficus sycomorus + Ficus burkei

Modified roots – Aerial Clinging roots Aerial roots to anchor plant to bark or branch of another plant e.g. Climbing plants and some epiphytes e.g. Philodendron monstera

Modified roots – Aerial Crampons Short adventitious roots for climbing = Crampons e.g. Hedera helix - Ivy

Modified roots–Aerial Pneumatophores In intertidal mangroves, roots above the tide – pneumatophores can “breathe” to allow oxygen to reach submerged roots in anaerobic soil

Modified roots – Strangler roots Strangler figs start as epiphytes, long aerial roots grow down to the ground to anchor the tree. As a tree it competes with its host for light + nutrients Eventually the mesh of roots strangle/smother + kill the host by crushing the secondary phloem

Modified roots – Photosynthetic roots some epiphytes e.g. Phalaenopsis Moth orchid

Modified roots –Contractile roots Plants with corms or bulbs (underground stems) have wiry roots that pull them deeper into the soil each season to keep them firmly underground. e.g. Gladiolus

Modified roots- Parasitic roots Stem parasites - epiphytes e.g. Tapinanthus oleifolius, mistletoe and Dodder, have special “roots” to penetrate the host plant to absorb water – Houstorium Root parasites – e.g. Striga, Dodder, Rafflesia

Stem structure – Dicotyledonous vs Monocotyledonous stem

Terminology

Modified stems – Stolons + Tendrils Stolons - Horizontal stem /runner e.g. Fragaria Roots + new shoot at each node Tendrils e.g. Vitis

Modified stems – Underground stems Rhizomes and Corms Rhizomes e.g. Iris, Phragmites Adventitious pull roots Survive unfavourable season Corms e.g. Gladiolus

Modified stems – Underground Tubers + Bulbs Tubers e.g. Solanum Potatoes Bulbs e.g. Allium, Onions, Crinum paludosum

Modified stems- Cladodes Fleshy stems that photosynthesize and store water New plant can grow vegetatively from cladode e.g. Euphorbia virosa, Opuntia spp. Spines are modified leaves, stiff, slender, sharp stuctures that develops from below the epidermis

Modified stems – Thorns Gymnosporia buxifolia Carissa bispinosa Thorns are modified stems that can have leaves + flowers on them – stiff, woody, sharp modified stems

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