[Music] hi and welcome back to three science lessons cold okay by the end of this video you should be able to describe the roles of the different tissues in a plant now I need to be clear here there's a lot of detail in this video and the best thing you can do is learn it it's very likely to be on you exam in the last few videos we've been looking at photosynthesis remember that photosynthesis takes place in leaves the leaf is a plant organ like any organ the leaf contains different tissues we've been looking at the functions of these tissues in this video this diagram shows you a cross section of a leaf in other words the least been sliced from top to bottom the top and bottom of the leaf are covered with a layer of very thin cells these are called epidermal cells and they form epidermal tissue so at the top of the leaf we've got the upper epidermis and at the bottom of the leaf we've got the lower epidermis the epidermis protects the surface of the leaf now the key adaptation of the upper epidermis is that it's transparent this allows light to pass through to the photosynthetic cells below the up epidermis is also covered with a thin layer of oily material called the waxy cuticle the waxy cuticle reduces the evaporation of water from the surface of the leaf and this helps to prevent the leaf from drying out the lower epidermis has tiny pores called stomata stomata allow carbon dioxide to enter the leaf and oxygen to leave stomata also help to control the amount of water vapor that can pass out of the leaf on either side of the stomata we find guard cells we'd be looking at the roles of stomata and guard cells in more detail in the next video at the top of the leaf we find the palisade mesophyll the palisade mesophyll consists of palisade cells palisade cells are packed full of chloroplasts remember that chloroplasts contain chlorophyll which absorbs the light energy needed photosynthesis now underneath the palisade mesophyll we find the spongy mesophyll the key feature of the spongy mesophyll is that it full of air spaces the air spaces allow carbon dioxide to diffuse from the stomata through the spongy mesophyll to the palisade cells like this oxygen also diffuses from the palisade cells through the spongy mesophyll to the stomata there are two other tissues in the leaf that we need to look at and these are the xylem and the phloem we've already seen the adaptations of these in the video on specialized plant cells xylem tissue transports water from the roots to the stem and the leaves some of the waters then used in photosynthesis the xylem also transports dissolved mineral ions these include magnesium which is used to make chlorophyll phloem tissue transports dissolved sugars produced by photosynthesis from the leaves to the rest of the plant the sugars can be used immediately for example glucose is used in respiration the sugars can also be stored for example a starch now the movement of sugars under the molecules through phloem tissue is called translocation and it's important that you learn that okay the final type of plant tissue we need to look at is called meristem tissue we find meristem tissue outgrowing tips for example shoots and roots meristem tissue contains stem cells these can differentiate into different types of plant tissue and again we looked at stem cells in a previous video remember you'll find plenty of questions on plant tissues in my ization workbook and you can gather by clicking on the link above ok so hopefully now you should be able to describe the roles of the different tissues in a plant you [Music]