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 a 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 those 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 site of 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 comes from the soil and is transported to the leaves by the roots and xylem as we'll see in the next video but the carbon dioxide diffuses directly into the leaf from the outside there through lots of little holes called stomata in fact each of the leaves will have thousands of these tomatoes and they're scattered through the lower epidermis which is the layer of hypodermal tissue once the carbon dioxide has diffused through these little holes it moves up the leaf and enters the spongy mesophyll 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 mesophyll layer this is where most of the photosynthesis happens and so the palisade cells are packed full of chloroplasts above this with 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 the chloroplasts in their 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 phloem now the main problem that leaves face is water loss the xylem which runs next to the phloem here continually brings water up from the roots for the palisade cells used 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 the stomata 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 stomata 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 stoma 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 a gap between them larger allowing more carbon hydroxide to diffuse through on the other hand when the plant is short of water the guard cells will lose water due to osmosis and they'll become flaccid this in effect closes the stomata meaning 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 night time when photosynthesis isn't taking place and they don't need carbon dioxide and the reason that most of the stomata are 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 meristem 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 we've actually already taken a look at meristems in our video on stem cells if you want to check it out we'll also put down the link to our active transport video which covers the structure and function of root hair cells and that's everything for this video if you enjoyed it then please do give us a like and we'll see you next time