Plant Classification• Plant bodies can be divided into two parts ROOTS and SHOOTS – Roots - below the ground – Shoots – above the ground (ie. Stems, flowers and leaves)• Plants can be divided into two groups, based on the structure of their roots and shoots: a) Monocots b) Dicots
Learning Goals• Be aware that many plants have a vascular system used to transport water and nutrients.• Be able to describe the purpose and structure of xylem and phloem.• Describe transpiration and translocation.
Distributing Materials• Plant cells need water, oxygen, certain nutrients and glucose just like animal cells. Photosynthetic cells also need carbon dioxide and sunlight.• Small plants such as mosses and liverworts will obtain all of these requirements by diffusion. These plants are non-vascular.
Vascular Plants• For large plants many cells are located large distances from the source of the nutrients they require and diffusion would be inadequate.• For example a plant obtains water and many nutrients from the soil. The leaves of trees are a long way away from the roots. Root cells require glucose but are very far from the leaves that produce it.• Larger plants therefore have evolved a transport system that moves such materials around the plant.
Xylem and Phloem• Plants have two transport systems: – Xylem transports water and nutrients up the plant from the soil – Phloem transports sugars throughout the plant• Xylem and Phloem consist of tubular pathways through which fluids flow through that are continuous with the roots, stems and leaves.• Xylem and Phloem together make up the vascular system of plants.
Xylem and Phloem• You can see the vascular system of many plants: the “veins” in blades of grasses and leaves and the stringy part of celery.• The arrangement of xylem and phloem is different in the various parts of a plant.
Vascular Tissue in the Root• Within the root the vascular tissue is located in the centre of the root and is known as the stele• The stele is enclosed by the endodermis
Vascular Tissue in the Stem• In the stem vascular tissue takes the form of bundles in dicot plants.• The xylem is located towards the inside of the stem• The phloem is located towards the outside of the stem
Vascular Tissue in the Leaves• Vascular tissue in the leaves also takes the form of bundles but these bundles run across the leaf as “veins”• Xylem forms the upper part of the bundle• Phloem forms the lower part of the bundle
Xylem• Xylem includes: – Fibres: have thick walls which act as supporting tissue – Parenchyma cells: living cells with thin walls that store materials and also provide support. They are found between the xylem vessels. – Xylem vessels: cells arranged on top of each other that form a column. The sides of these cells are thickened with lignen. These cells die and loose their nucleus and cytoplasm as well as their end walls becoming one long tube. – Tracheids: long dead cells that have tapered ends. The end walls of these cells are not fully disintegrated so the ends of these cells form “pits”. Water passes from one “pit” to the next.
Xylem• The tracheids of xylem can appear to have spirals due to the lignin. This lignin can form in different ways to form different patterns.
Phloem• Phloem consists of: – Sieve tubes: consist of many cells arranged above each other to form a tube. These cells are alive and their end walls are perforated forming a “sieve plate” – Companion cells: living cells that sit alongside the sieve tube cells. Involved in moving sugars into and out of the sieve tube cells. These cells are linked to the sieve cells by plasmodesmata (strands of the cytoplasm). – Phloem also has parenchymal cells and support fibres.
Transpiration• Transpiration (loss of water from the leaves) drives the upward movement of xylem “sap”• As water is cohesive (sticks together) therefore the removal of water from the surface of the leaves draws water from the xylem vessels to replace this water. This in turn draws water from xylem vessels in the stem and eventually the roots causing net upward movement of water.
Translocation• Translocation is the movement of organic compounds produced by the plant such as carbohydrates and amino acids.• Translocation occurs through the phloem.• Translocation occurs downwards as roots are often “sinks” for storing carbohydrates.• Translocation can also occur upwards as new shoots and growth will need a supply of carbs.• Translocation is an active process requiring energy.
Translocation• Movement of fluids through phloem occurs due to pressure gradients.• Pressure will build up in a area in which a material is being actively pumped into the sieve tubes. It will be decreased in the area at which this material is being used.• This difference in pressure will cause the net movement of material to the place that it is required.