Plant tissues and the structure of a leaf

Picture painting clear nail varnish over one face of a leaf, on a healthy houseplant. Which face would harm the leaf more, the top or the underside? Most people pick the top, the side facing the light. But paint two leaves, and a week or two later the leaf varnished underneath is pale and failing, while the other looks fine. Open the leaf up layer by layer, and the underside explains itself.

A leaf is an organ, a part of the plant made of several tissues working together. A tissue is a group of similar cells that do one job. If those levels feel shaky, the video on levels of organisation covers them. With the stem and the roots, the leaves form the plant's transport system, which moves water and sugar around it. A plant also has meristem tissue at the growing tips of its shoots and roots, where new cells are made. For photosynthesis, a leaf has to catch light, take in carbon dioxide and receive water, and it lets oxygen back out. Its shape helps with the first job. A leaf is broad and flat, which gives it a large surface to catch the light. Should a leaf be thick or thin, if gases have to reach every cell inside it? Thin. Carbon dioxide then has only a short way to travel, from the air outside to the furthest cell. Whatever your course, the videos after this one are built on this leaf, so stay with it.

Now slice across the leaf and look at the cut edge. From top to bottom there are five parts, and each one is built for its job. On top is the upper epidermis, a thin, see-through layer of cells. Light passes straight through it to the cells beneath. Just under it is the palisade mesophyll. Its cells are tall and packed tightly together, and they are crowded with chloroplasts, the parts of a cell where photosynthesis happens. Packed tight near the top, they catch most of the light that gets in. That makes the palisade the main layer for photosynthesis. Now the layer below. It must get gases to every cell: what do its cells need between them? Gaps. This is the spongy mesophyll: rounder cells, loosely packed, with air spaces between them, so gases can move through to every cell. These cells have chloroplasts too, so they photosynthesise as well, only less. A different leaf shows long, upright cells packed side by side. Which layer is that? The palisade mesophyll, where the light is caught. Round, loose cells with air spaces would be the spongy layer, where gases move. One examiner's report on a Higher paper records that "Confusion between palisade and spongy mesophyll tissues was also frequent." In plain words, students swapped the two layers. The way to avoid it is to give each layer its position and its job, every time you name it. Through the middle of the leaf runs a vein, a bundle of two kinds of tube. Xylem brings water into the leaf, and phloem carries dissolved sugars out of it. What those tubes are made of is in the video on root hair cells, xylem and phloem. At the bottom is the lower epidermis, and dotted through it are tiny pores called stomata. One pore on its own is a stoma. Around each stoma sits a pair of guard cells. The guard cells are the two curved cells, and the stoma is the hole between them. How they open and close it is in the video on transpiration, translocation and the stomata.

Now for what moves in and out of a leaf, starting with the water it needs. How does water get into a leaf: through the stomata, the surface, or up the vein? Up the vein. Water climbs from the roots through the xylem, then moves out into the mesophyll cells. It never comes in through the surface or a stoma. Carbon dioxide comes in through the stomata by diffusion. It moves from the air outside, where there is more of it, into the air spaces, where there is less. Diffusion itself is in the video on diffusion and active transport across the cell membrane. Oxygen made in photosynthesis goes the other way. It diffuses out through the stomata, from more inside the leaf to less outside. To find where most stomata sit, a student counts them on an ivy leaf, in three fields of view on each surface. The upper surface gives counts of two, nought and one. The lower surface gives thirty-one, twenty-seven and twenty-nine. Work out the mean count for each surface, upper and lower. For the upper surface, add two, nought and one to get three. Divide by three, and the mean is one stoma per field of view. For the lower surface, thirty-one plus twenty-seven plus twenty-nine is eighty-seven. Divide by three, and the mean is twenty-nine. So the lower surface has a mean of twenty-nine stomata per field of view, against one on the upper surface. That is about twenty-nine times as many. Now the reason: why would a leaf keep most of its stomata on the underside? The underside is shaded and cooler, so pores there let out less water. And the spongy layer's air spaces sit right above it, so gas coming in reaches every cell. That explains the varnish. Paint the underside and you seal most of the stomata, so carbon dioxide cannot get in and the leaf cannot photosynthesise. Paint the top, and almost nothing is blocked.

Time to put it all together, with five features of a leaf and the names taken off. For each one, name the tissue, say whether its job is catching light, letting gases move, or carrying water and sugar, and say why its structure suits that job. Pause and name all five, with a job and a reason for each. Tall cells packed tight under the top, full of chloroplasts: the palisade mesophyll. It catches light, because its cells sit where the light arrives and are crowded with chloroplasts. Rounder cells with gaps between them: the spongy mesophyll. It lets gases move, because the air spaces carry carbon dioxide to every cell and let oxygen out. Pores on the underside, each between two curved cells: the stomata and their guard cells. They let gases move, because carbon dioxide gets in and oxygen gets out through them. A thin, see-through layer over the top: the upper epidermis. It serves the light, because light passes straight through it to the palisade. A bundle of tubes through the middle: the vein. It carries water in through the xylem, and dissolved sugars out through the phloem. Now mark a student's sentence: the spongy mesophyll is where the leaf stores food, and water gets into the leaf through the stomata. Which half of that sentence is right: the first, the second, or neither? Neither half is right. Both are wrong, and in different ways. The spongy mesophyll is not a store: its loose cells and air spaces let gases move through the leaf. And water does not come in through the stomata. It comes up the xylem in the vein, from the roots. A second examiner's report, on another Higher paper, puts numbers on the spongy mesophyll. "Half of the students scored at least one mark, usually for saying that the spongy mesophyll is adapted for its function by having air spaces." Then: "Only a fifth of students went on to explain the function of the air spaces, to score two marks." The same report goes on: "There was quite a lot of confusion regarding how water enters a plant, many thinking that it entered via the leaves. Some students thought the spongy mesophyll was a storage tissue." In plain words, many students named the air spaces and stopped there. The fix is to give every feature its job, as each of those five answers did: air spaces, so that gases can reach every cell.

Three questions to close, and the last is a leaf you have not met yet. Which layer of a leaf catches most of the light, and what lets it? The palisade mesophyll. Its tall cells sit packed tight near the top, crowded with chloroplasts to catch the light. Now the gases. If a leaf grew twice as thick, which of its cells would struggle, and why? The cells furthest from the stomata. Carbon dioxide would have much further to diffuse from the air outside, so less of it would reach them. Here's a different leaf. A water lily's leaf lies flat on a pond, with its underside resting on the water. Where would you expect a water lily's stomata to be, and why? On top, facing the air. Carbon dioxide can only diffuse in where there is air to bring it. And the two varnished leaves come down to the same idea. Varnish underneath sealed most of the stomata and shut out the carbon dioxide, while varnish on top blocked almost nothing.

That's the chapter so far, with this video last. If you could teach this one to a friend, tap the thumbs up, and you'll know you never need to watch it again. If it hasn't clicked yet, save the playlist; the rest of the chapter keeps using this leaf, so it comes back round.

Next in the chapter: root hair cells, xylem and phloem. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh five, oh two.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.