1. School of Sciences, Lautoka Campus BIO509 Lecture 23: Roots, stems and leaves

2. Learning outcomes Know the different parts of a plant.
Know the anatomical structures of roots, stems and leaves.
Learn the different types of modifications of roots, stems and leaves.
Understand and appreciate the human and ecological relevance of various modifications.

3. Parts of a plant

4. Three basic organs:
roots, stems, and leaves.
They are organized into a root system and a shoot system (consisting of stems and leaves).
Shoot and root systems are interdependent

5. Roots Absorption of water and minerals.
Anchorage of plants in one place for the entire life.
Conduction and transport of water and mineral to and from the soil.
Storage of food and energy reserves.
Absorption occurs near root tips, mostly in root hairs (extension of epidermal cell)

6. Root structure Three regions: Region of cell division.
- composed of apical meristem tissue, region of active cell division.
Region of elongation.
- cells increase in size.
Region of maturation.
- cell mature and differentiate into various types of tissues.

7. Zone of cell division
Root cap serves to protect growing point of the root.
Apical meristem where active cell division takes place.
Divides into three meristematic regions
- Protoderm, the outer laye r giving rise to the epidermis.
- Ground meristem, which gives rise to the parenchyma of the cortex.
- Procambium, forming xylem and phloem in the vascular cylinder.

9. Zone of cell elongation
New cells formed by mitosis elongate and their vacuoles merge.
Rest of root remains stationary, as only these first three zones push through the soil.
Eventually form mature structures in the zone of differentiation.

10. Region of Differentiation
Also called root hair zone.
Root hairs greatly increase the surface area for water absorption.
Numerous and appear as a fine down, seldom more than 1 cm long.
Cuticle on epidermal cells relatively thin to allow water absorption.
Zone remains fairly constant in length as root below it grows downward and the new area becomes the zone of root hairs.
Differentiates into distinctive cell types and tissues.

11. Cell types in roots Cortex cells
Cortex is composed of parenchyma and lies between epidermis and inner tissues.
Presence of an endodermis - single layer of cells with walls impregnated with suberin.
Endodermis found in all roots of angiosperms except three species.

12. Cell types in roots Casparian strips
Suberin bands on radial and transverse walls of endodermis.
Composed mostly of suberin that is impermeable to water.
Prevent water from passing through otherwise porous cell walls
Directs water to go through the endodermis plasma membrane, which regulates it.

14. Cell types in roots
Pericycle – cells lying against the inner boundary of the endodermis.

15. Cell types in roots Vascular cylinder lies inside endodermis.
The arrangement of bundles in the root is called the stele.
Mostly water and food conduction cells – xylem and phloem.

16. Dicot root in Cross Section

17. Monocot Root in Cross Section

18. Modified Roots
Apart from absorption of water and minerals, some roots modified for specific functions.
- Food storage.
- Photosynthesis.
- Support.
- Water storage and retention.
- Gas exchange.
- Parasitic roots.
- Storage of nitrogen to nitrate converting bacteria.

19. Storage roots store food mostly as carbohydrates, e. g
Storage roots store food mostly as carbohydrates, e.g. yams and sweet potatoes
Usually stored in extra parenchyma cells, causing root swelling.
Carrots, beets, and turnips are actually part stem and part root

20. Some plants store water in their roots, e.g., some cactus.
Used to supply the plant in the dry season

21. Adventitious roots grow from places other than from the underground root (e.g., form the stem)
They are used for various purposes, e.g. support, vegetative reproduction, photosynthesis, clinging to rocks and trees, support, etc.

22. Propagative Roots
Suckers grow from roots running near soil surface and produce new plants by vegetative reproduction.
Common in fruit trees, such as apples, cherries, bananas, and pears, breadfruit

23. Pneumatophores
Aeration roots used for gas exchange, particularly in wetland plants, since water contains relative little oxygen.
The roots may grow down from the stems (as adventitious roots also called prop roots) or up from the roots as knobs or spikes sticking up from the soil.
Typically covered with lenticels and filled with spongy tissue
Typical of mangrove plants.

24. Rhizophora are mangrove species have pneumatophores that serve for aeration and support

25. Climbing Roots used to cling to trees or rock (e.g., ivy)
These stick to the bark or rock surface.

26. Banyan trees (Ficus spp
Banyan trees (Ficus spp.), otherwise known as strangler figs, start out an epiphyte, e.g. Ficus obliqua.
They send aerial roots down the trunk, which eventually reach the ground and root.
Eventually the tree is surrounded and killed, since it cannot grow with the strangler all the way around its trunk.

27. Buttress Roots
Give rainforest trees stability and protection from windfall from storms
Prop roots help to support trees as well, but are separate from the trunk, like in Myristica (nutmeg)

28. Parasitic roots
Haustoria of parasitic plants invade host to obtain water and minerals mostly, since most can produce their own food

29. Root Nodules
Atmospheric nitrogen is 80% of air, but cannot be used directly by plants
Some bacteria produce enzymes that convert N to nitrates and other N compounds
Some plants have nodules on their roots that house bacteria
Especially members of the pea family Fabaceae

30. Questions??