Plant Tissues, organs and systems
plant tissues
epidermal tissue - covers the whole plant and are covered with a waxy cuticle which can help to reduce water loss by evaporation
palisade mesophyll tissue - this is the part of the leaf where most photosynthesis happens and has lots of chloroplasts so is located near the top where they can get most light
spongy mesophyll tissue - in the leaf and contains big air spaces to allow gases to diffuse in and out of cells
xylem and phloem - transport water, minerals and food around the plant and they form a network of vascular bundles, they also help support the structure
meristem tissue - found at the growing tips of shoots and roots and can change into lots of different types of plant cell allowing the plant to grow
upper epidermis - transparent so that the light can pass through to the palisade layer
phloem
made of columns of elongated living cells with small pores in the end wall to allow sap to flow through
they transport food substances (sugar) made in leaves to the rest of the plant for immediate use or storage
the transport goes in both directions
this is called translocation
xylem
made of dead cells joined end to end with no end walls between them and a hole down the middle, they are strengthened with a material called lignin
they carry water and mineral ions from the roots to the stem and leaves
the movement of water from the roots through the xylem and out of the leaves is called transpiration stream
factors that affect the rate of transpiration
light intensity - the brighter the light the greater the transpiration rate
root hair cells
plants absorb water from the soil by osmosis, root hair cells are adapted for this by having a large surface area to speed up osmosis
the absorbed water is transported through the roots to the rest of the plant where it is used for different purposes such as cooling the leaves by evaporation, transporting minerals around the plant or a reactant used in photosynthesis
temperature - the warmer it is the faster transpiration happens
humidity - the drier the air around the leaf the faster transpiration happens
air flow - the better the air flow around the leaf the greater the transpiration rate
stomata and guard cells
guard cells are adapted to open and close stomata
they have a kidney shape which opens and closes the stomata
when the plant has lots of water the guard cells fill with it and go plump, this makes the stomata open so gases can be exchanged for photosynthesis
when the plant is short of water the guard cells lose water and become flaccid making the stomata close which helps to stop too much water from escaping
thin outer walls and thickened inner walls make the opening and closing work
they are sensitive to light and close at night to save water without losing out on photosynthesis
more stomata are on the underside of the leaf as it is shaded and cooler so less water is lost