Leaves, Flowers, Fruits and Seeds

A leaf is usually flat and green. The flat, wide part of a leaf is called the blade, or lamina. Inside the blade, we can clearly see thin lines called veins. In many leaves, one thick vein runs through the middle, called the midrib, and thinner veins branch out from it. A small stalk called the petiole joins the leaf to the stem or branch. Some leaves, mostly on monocot plants, do not have a petiole, instead a part of the leaf wraps around the stem, called a sheath.

A leaf contains a green colouring called chlorophyll, which gives it its green colour and lets it make food using sunlight, water, and carbon dioxide. This food-making process is called photosynthesis, and because of this, a leaf is often called the food factory of the plant. On the surface of a leaf are tiny pores called stomata, which let gases move in and out during photosynthesis and breathing. Water vapour also escapes through the stomata in a process called transpiration.

A leaf has to collect two things that are spread very thinly through the air around it: sunlight, and carbon dioxide. Neither can be gathered from a point. Both have to be caught over an area. That is why a leaf is flat and wide, and it is the same idea you met in the last two topics. A cell stays small so its surface can serve its inside. A root grows thousands of hairs to pack surface into a small space. A leaf spreads itself out into a sheet. Three completely different parts of a plant, all solving the same problem.

Being thin matters just as much. Light has to reach the cells inside the leaf, not just the ones on top, and gases have to travel from the stomata to every cell that needs them. In a thick leaf the middle would sit in shadow and the gases would never arrive. Thin and flat together is not an accident of shape, it is the answer to the question the leaf exists to solve.

Stand under a large tree and look straight up. The leaves do not sit on top of one another. They are arranged so that each one finds a gap of sky, filling the space like pieces of a puzzle, because a leaf sitting in another leaf's shadow is a leaf earning nothing. Nobody arranged them. Plants that grow their leaves that way simply do better than plants that do not.

Stomata create a problem the plant can never fully escape. Carbon dioxide has to get in, and the only way in is through an open pore. But an open pore also lets water vapour out. So every time a leaf opens its stomata to feed itself, it loses water, and on a hot dry day it can lose a great deal.

This is why many plants close their stomata around the hottest part of the day. They stop making food for a while, because keeping the water they already have matters more. It is also why a plant left unwatered on a sunny windowsill droops far faster than the same plant in a shady corner. And it explains the cactus from the last topic. Its leaves became spines precisely to cut this loss, and the job of making food moved to the green stem instead.

A flower is the part of a plant where seeds are made. The male part of a flower is called the stamen, which makes tiny grains called pollen. The female part of a flower is called the pistil, its sticky tip is called the stigma, and it leads down to the ovary, which holds the ovule. When pollen travels from the stamen to the stigma, this is called pollination. Insects and wind often help carry pollen from one flower to another. After pollination, the male and female cells join together inside the ovule, this joining is called fertilisation. After fertilisation, the ovule becomes a seed, and the ovary grows into a fruit.

Pollen has to get from one flower to another, and a plant cannot walk it there. So it has two choices: pay something to carry it, or throw it into the wind and hope. Which choice a plant makes decides what its flowers look like, and once you know this you can look at any flower and say how it is pollinated.

A flower is not decorated for us. Its colour and scent are an advertisement aimed at an insect, and the nectar is the fee. Look at a maize plant and you will find no petals worth the name, because there is nobody to advertise to. Instead it releases clouds of dry pollen that drift wherever the wind takes them. Almost every grain is lost, which is exactly why it makes so many, and it is also why grass and pine pollen make so many people sneeze at certain times of year.

A fruit forms from the ovary of a flower after fertilisation, and it protects the seeds inside. A fruit has three layers, together called the pericarp. The outermost layer is the exocarp, the thick middle layer is the mesocarp, and the innermost layer, closest to the seeds, is the endocarp. Most flowering plants reproduce from seeds. The outer covering of a seed is called the seed coat, and it protects the parts inside. Inside a seed is the embryo, along with a food store called the endosperm and one or two seed leaves called cotyledons. The embryo has a tiny root called the radicle and a tiny shoot called the plumule.

That last row catches nearly everybody. A mango stone is not the seed. It is the endocarp, the innermost layer of the fruit, hardened into armour. Crack one open and the real seed is sitting inside, soft and pale. The same is true of a peach, and of a plum. Once you have seen inside one stone you never mix the two up again.

A seed that falls straight down lands in its parent's shadow, in soil the parent is already using, and it does badly. A plant cannot carry its seeds anywhere, so it does the next best thing. It wraps them in something an animal wants, and lets the animal do the carrying.

That is what sweet flesh is for. It is payment. A bird or a monkey eats the fruit, moves away, and the seed is dropped somewhere else, often with a helping of natural fertiliser. Many seeds have coats tough enough to pass right through an animal unharmed, which is not a coincidence at all. Even a mango works this way, in a rougher fashion. The stone is too large to swallow, but the flesh is stripped off and the stone dropped some distance from the tree, which is still further than it would have fallen.

This also explains unripe fruit. A green mango is sour, hard, and the same colour as the leaves around it, and none of that is an accident. The seed inside is not finished yet, so being eaten now would waste it. Only when the seed is ready does the fruit turn colour, soften, and sweeten. The fruit is not becoming ripe for you. It is putting up a sign that says the seed is ready to travel.

A dicot seed, like a bean, has two cotyledons and a small endosperm. A monocot seed, like maize, has only one cotyledon and a larger endosperm. When a seed has enough warmth and water, it starts to grow, this is called germination. The radicle grows down into the soil and becomes the root, while the plumule grows up and becomes the shoot. During germination, the cotyledons provide food to the growing embryo until the plant can make its own food through photosynthesis.

Notice which comes first. The radicle goes down before the plumule comes up, every time, and there is a good reason for the order. A shoot that appeared first would sit in the sun with nothing to drink. Going down first means the seedling has water and an anchor before it risks the open air. A seed also does this in complete darkness underground, with no way of seeing which way is up, and it still gets it right. Roots grow towards gravity and shoots grow away from it, so the seed can be planted upside down and the seedling will still sort itself out.