5.3: Plant Tissues
3A: Plant Tissues
Recall from grade ten science that a tissue is a collection of cells with a similar function.
Vascular
Ground
Meristematic
Dermal
Vascular Tissue
Phloem - Transports sugars from the leaves to the roots.
Xylem - Transports water and minerals from the roots to the leaves.
Phloem
Sieve tube cells make up a part of the phloem tissue and are interconnected with perforations (see diagram below).
Sieve tube cells lack nuclei at maturity, but are still living.
Companion cells make up the other part of the phloem, and actively transport sugars in and out of the sieve tubes.
Notice in the image above how the sieve tube cells are connected by perforated membranes to allow sugar to pass from one cell to another? Also notice how sieve tube cells (have no nuclei) are surrounded by companion cells (have nuclei).
Xylem
Xylem is made up of vessel element cells and tracheids.
Vessel elements and tracheids are dead cells arranged end to end (perfora) and side by side (pit).
Tracheids are smaller in diameter and length than vessel elements.
The diagram above shows how both vessel elements and tracheids are connected by perforations on their end plates. Xylem are also connected on their sides, by pits, to allow additional horizontal water transport. The horizontal transport of water allows water to enter the phloem to help push sugars down to the roots.
Ground Tissue
Parenchyma (alive & thin-celled)
Structural support.
Food and water storage.
Photosynthetic.
Parenchyma cells, pictured above, have thin primary cell walls. These cells are soft to eat, and are the site of photosynthesis in plants.
Collenchyma (alive & thick-celled)
Structural support where rapid growth occurs, have thicker primary cell walls than parenchyma, but are not as solid or hard as sclerenchyma.
Compare the above image of collenchyma to the image of parenchyma from the previous example. Notice how the primary cell wall is much thicker? These cells are still living, but offer more structural support to the plant than parenchyma. They can be tough to chew when eaten.
Sclerenchyma (dead & thick-celled)
Structural support.
The sclerenchyma pictured above has a thickened secondary cell wall underneath the primary cell wall. These cells are the best of the ground tissue cells, and are no longer living.
Meristematic Tissue
Plant growth by mitosis occurs only at the meristems.
Primary Growth
Growth in length of the plant (plant growing up, and roots growing down into the soil).
Occurs at apical meristems (tips of shoot, leaves and roots).
Secondary Growth
Growth in width.of the plant.
Recall the image above of the tree cross section - pay close attention to the cork cambium and the vascular cambium. These are the meristem tissues that cause secondary growth in plants.
Vascular cambium (between xylem & phloem in dicots).
Grows new xylem & phloem.
Cork cambium (outer layer of woody stem).
Grows new cork
Dermal Tissue
Epidermis
One cell thick.
Outer covering of the plant.
Waterproofing (via cuticle) on leaves and stems.
Protection from bacteria on roots.
Cork
More than 3 cell layers thick.
Waterproofs stems only.
3B: Water and Food Transport
Water Transport
Plants transfer water from the roots throughout the plant by using a combination of two processes:
Root Pressure
Leaf Pull
Root Pressure
Root hairs absorb minerals via active transport. Active transport means that this process uses energy.
Glucose from photosynthesis in the leaves is stored in the roots, and is used to produce energy through cellular respiration.
Minerals pumped from outside of the root, through the epidermis through the cortex, through the endodermis and then into the xylem.
The flow of minerals into the xylem in the roots sets up a hypertonic condition, meaning that there is more solute (minerals) in the xylem than outside of the xylem, so water rushes into the xylem to try to make the concentration of minerals outside of the xylem, equal to that inside the xylem.
Water moves into the xylem through the process of osmosis, which is the movement of water across a semi-permeable membrane.
All of these water and mineral molecules creates a high pressure area in the xylem near the roots. Water moves from high pressure in the roots, towards the relatively lower pressure area in the leaves.
The image above shows root hairs developing on the tap root of a newly formed bean plant. These hairs will use energy to transport minerals from the soil into the plant's xylem. Water will follow the minerals into the plant's xylem, which creates a high pressure area of water in the root.
The image above shows how minerals enter cells with root hairs (which is active transport, and takes energy), the minerals then flow through the cortex and endodermis and into the xylem in the vascular cylinder.
Leaf Pull (Transpiration Pull)
Evaporation of water “pulls” on adjacent water molecules using adhesion and cohesion.
Water molecules are polar, meaning that they have a positive and negative pole. These form weak attractive forces between adjacent water molecules.
When water molecules bond to the sides of the xylem, this is called "adhesion", which is simply one substance (water), binding to the surface of another substance (the xylem).
Water molecules also bond to each other, allowing them to further climb up the xylem against gravity. This bonding between water molecules is called "cohesion".
As water evaporates from the leaves of a plant through the stomata, it creates an area in the plant where there is now less water molecules than in the roots. Therefore, the leaves have a lower pressure from water than the roots. Molecules naturally move from high to low pressure, so the higher pressure water in the roots moves to the lower pressure area in the leaves via adhesion and cohesion in a process called leaf pull.
Food Transport
Pressure-Flow Theory
Sugar is created by photosynthesis in the leaves and is actively transported from leaves into phloem. (again, active transport means that it takes energy).
The phloem in the leaves is now hypertonic relative to the xylem which is beside the phloem. Water rushes into the phloem in the leaves from the xylem via osmosis.
As water rushes into the phloem at the leaves, there is more water in the phloem of the leaves than in the phloem in the roots. Therefore, the sugary, high pressure water in the phloem in the leaves, rushes to the low water pressure area in the phloem in the roots.
In other words, phloem sap is pushed down and stored in roots.
The above diagram represents the pressure flow theory of how sugar makes it from the leaves to be stored in the roots in plants. Turgor pressure refers to the water pressure within plant cells. Plant cells with a high turgor pressure have more water inside of them than cells with a low turgor pressure.