just like in animals plants have different levels of organization with lots of similar cells combining to form tissues different types of tissues combining to form organs and then multiple organs combining to form organ systems to see how these fit together we're going to take a look at the structure of a leaf which itself is an organ and along with the stem and Roots it forms an organ system that's function is a transport of substances around the plant this image here is a cross-section of a leaf as though you sliced it from top to bottom and are looking inside before we explore these structures though it's worth considering its function leaves are the cytop photosynthesis which we'll cover in another video but basically it requires lots of carbon dioxide and water in order to make sugars the water water comes from the soil and is transported to the leaves by the roots and xylm as we'll see in the next video but the carbon dioxide diffuses directly into the leaf from the outside air through lots of little holes called stamata in fact each of the leaves will have thousands of these tomata and they're scattered through the lower epidermis which is a layer of epidermal tissue once the carbon dioxide has diffused through these little holes it moves up the leaf and enters the spongy misail tissue which has lots of air gaps between the cells so that the gas can easily diffuse through to the next layer which we call the palisade misop layer this is where most of the photosynthesis happens and so the Palisade cells are packed full of chloroplasts above this we have the upper epidermis which is another layer of epidermal tissue these cells are almost transparent though as the sunlight needs to be able to pass through them to get to the chloroplasts in the Palisade cells beneath and once photosynthesis has finally produced the sugar molecules they're carried to the rest of the plant by these green tubes here which we call the flm now the main problem that leaves face is water loss the xylm which runs next to the flum here continually brings water up from the roots for the Palisade cells use in photosynthesis but that water can be lost from both the top and bottom of the leaf to reduce this water loss the leaf has a waxy cuticle on top which is basically a thin waterproof layer of lipids that the water can't get through when it comes to the bottom of the leaf the main problem is this tomato as the water will diffuse out of any gaps really easily the leaves need these holes though so they can get the carbon dioxide that they need as a compromise the leaves keep their stamato open for as short a time as possible so they can maximize carbon dioxide absorption but minimize water loss in order to achieve this delicate balance each sto which is what we call a single stomata is formed from the gap between two guard cells when the plant has lots of water so it doesn't need to worry so much about conserving it the guard cells will be well hydrated which we call turgid and this makes the gap between them larger allowing more carbon dioxide to diffuse through on the other hand when the plant is short of water the guard cell will lose water due to osmosis and they'll become flaccid this in effect closes the stamata mean that the plant no longer takes in carbon dioxide but more importantly it will conserve its water vapor another adaptation is that the guard cells are sensitive to light so they close at nighttime when photosynthesis isn't taking place and they don't need carbon dioxide and the reason that most of the Stato on the underside of the leaves rather than the top is that the lower surface is more shaded which makes it cooler and so it means that less water will evaporate one last tissue we need to mention is Merry stem tissue which is basically the plant form of stem cells they're found at the growing tips of the roots and shoots and can differentiate into loads of different cell types so that the plant can grow if you haven't heard yet you can find all of our videos on our website it Cognito dog you'll also find questions flashcards exam style questions and past papers and we track all of your progress so that you always know what to study next so sign up for free by clicking here or browse our playlist here on YouTube