The world is divided up into “biomes” – meaning “bioclimatic zones”. In each biome the plants, animals, insects, and people live in a certain type of climate, or weather system. The temperature, the amount of rainfall and the amount of sunlight all vary and the flora and fauna have evolved so that they can survive in their own particular conditions, such as tropical rainforests, deserts and mountains. People can live almost anywhere in the world, because they can adjust the conditions to suit themselves – by wearing more or fewer clothes, by heating or cooling their homes – but plants have had to change themselves instead.
They have grown in the areas of the world where there is the most heat and the least rainfall so they have adapted to a desert biome.
Plants have developed over millions of years and they have evolved in many different ways according to where they live. Mesophytes are plants that grow in places where there is average, or normal, amount of water for the plants to drink. Water plants, or hydrophytes, have evolved to live in wet places, and succulent plants, or xerophytes, have evolved to live in dry places.
Every plant is made up of roots below the surface of the soil and a shoot above the surface. The shoot has a stem and leaves. The roots feed the plant by sucking up water, food and minerals from the soil. The leaves make food for the plant by absorbing sunlight and turning it into sugar so that the plant can grow – a process known as photosynthesis. There are special cells in the leaves which contain chlorophyll and the plant uses these to make food. Chlorophyll is green, and this gives the leaves their green colour.
A ‘normal’ plant (a mesophyte) grows in a place where it can get sunlight and rain water regularly. They have leaves that are thin and flat. This is so that the leaves have a big surface area to catch all the sunlight they can.
Succulent plants grow in dry areas where they get lots of sunshine but they have to go for a long time without any water to drink. Succulent plants have roots, stems and leaves too but have changed them in clever ways to stop them burning up when they get too much sunlight and so that they can store any rainwater when it does come. They have also slowed down transpiration, which is the speed that the water empties out of the plant.
Succulents are called leaf, stem or root succulents according to how they have changed.
In stem succulents (such as cacti), the stems are green because they use their stems instead of the leaves to make their food from the sun by photosynthesis. This means that the stems have chlorophyll in them which therefore gives them their green colour.
Cacti are the best known of the stem succulent plants. The stems grow in simple shapes like columns and spheres, and this is where they store the water. They often have ribs and tubercles, these act like a concertina and can spread out and fold back in again depending on how much water the plant is holding inside it.
The leaves have changed into spines and the plants have a thick covering of these to protect them from the heat and from animals that might eat them.
On a cactus plant the spines always grow from an area of the plant called the areole; this is a furry part on the cactus where the spines and buds grow from. In a normal plant this would be a side shoot (lateral shoot). Of all the succulent plants only cacti have got areoles.
In a stem succulent the outside skin, or epidermis, has grown thicker and is covered in a thick waxy material called the cuticle which protects the insides of the plant. The inside is full of special tissue for storing water. This is a sort of packing material called parenchyma. In the middle there is a column reaching up from the roots containing vascular tissue, which lets the water travel up from the roots and through the plant.
There are some very unusual plants called pereskias which look like leafy bushes but are actually cacti. They show how the very earliest cacti looked almost like other ‘ordinary’ plants with stems and leaves.
Although the below photograph of Pereskia aculeata does not look like what we think of as a cactus, it has actually got areoles and spines just like ‘normal’ cacti. This is the very first cactus and it shows the very beginning of how cacti gradually changed to cope with heat and sunshine. If the pereskia gets very dry, all of its leaves will drop off, but the stem stays healthy and new leaves will grow when the water comes again.
Over time cacti changed until they did not look like ordinary plants at all. Mammillarias are a good example of spherically shaped cacti, while the Cleistocactus straussii has long slender columns. Some columnar cacti, like espostoa, have got a white woolly coat for protection as they grow in areas where the temperature can fall quite low at night.
But sometimes cacti have strange shapes for different reasons. The ‘cristate’ or wavy surfaced mammillaria has not grown into that strange twisted shape because of evolution. No-one quite knows why they become deformed like this – damage to the growing point or a virus infection are suspected as possible cause – but people like to collect them because they look so unusual. Cristate plants will actually try to change back to normal, and they will sometimes make new growth that look just like an ordinary cactus. So to keep a cristate plant looking good – or perhaps I should say looking bad! – it is important to keep removing any normal growth, or the plant will revert back to being its good rather than its evil twin!
You may have seen Christmas cacti which are grown for the mass of attractive flowers they produce. They don’t look like ‘ordinary’ cacti, but if you look carefully, you will see that it has the areoles and spines, though they are much less noticeable. Christmas cacti evolved to grow in forests, where they root in the debris which catches in the crooks of trees where the branches join the trunks and they hang down to catch the light. Because of this they have a small root system, their stems look like jointed leaves and they are fat and wide to get the maximum light in the shady forest.
These plants are called epiphytes. There are other epiphytic or tree-growing plants including epiphyllums (also known as orchid cacti) which are grown for their big flowers. They often have something called air roots or aerial roots. They are just like ordinary roots, but they take the water out of the air to help the plant to get even more water and sometimes they can even perform photosynthesis. These roots also help them to fasten themselves onto the tree branches and into pockets of dead leaves caught in the crooks of the branches.
There is a concept called parallel evolution or convergent evolution. In succulent plants this means that plants look very similar but they are not related in any way. They have just developed similar ways to cope with the strong light and lack of water.
Cacti grew and changed over many many years in North and South America, but there are some very similar looking plants which come from South Africa. These are called euphorbias. These are stem succulents too and they grow in columns and balls just like cacti but their leaves developed differently. Instead of forming areoles with spines, their leaves have turned into little thorns, or into very, very tiny leaves which often fall off leaving only the stem behind when it is very dry. These tiny leaves are known as ‘vestigial’ leaves as they are ‘vestiges’ or leftovers from big leaves.
Leaf succulents are plants whose leaves have become very fat so that they can store extra water. Sometimes the plant looks quite ordinary apart from its fat leaves. But the leaves have got a thicker skin on the outside to hold the water inside them and they have a waxy or furry surface to stop them from losing water so quickly.
Sometimes they grow in a rosette shape like a flower – a good shape for collecting what water there is. In other cases, succulents have developed tight, interlocking and overlapping structures, forming columns.
Haworthias have also got fattened leaves and a rosette shape. Lots of succulents have fat leaves like this, which have become thickened, and the leaves have a glossy, water retentive surface.
The adromischus has got thick leathery leaves with an oval shape to hold as much water as possible. Notice too how this has camouflaged itself with its mottled green and purple pattern to stop the animals from eating it.
Amongst the leaf succulents sometimes the leaves have become simplified into a divided pair of leaves which look just like pebbles and because of this they are called “living stones”. They grow in the very hot desert areas of South Africa and they have evolved to look just like stones, so they are camouflaged from grazing animals. They can pull themselves right down into the soil to protect themselves from the blazing sunshine. They store water in their large leaves and grow by shedding their skin like a snake, with the new, bigger plant coming out from the dried skin of the old plant.
Other living stones have developed a special structure on the surface, to make them look even more like stones or stone chippings. Look at the aloinopsis below and you will see that it has a stone-like surface which is known as ‘calcification’, so that the animals will not see it and eat it.
Some of the living stones have partial ‘windows’ in the top of the leaves, with transparent sections amongst dots and lines of thicker material.
In the desert they grow almost completely hidden in the soil, with only the transparent tops of the leaves peeping out, so that they can get light to the bottom of the leaves. Lithops do this, and so do other living stones such as opthalmophyllums.
Haworthias also have these windows to make sure that the light gets through the tight overlapping leaves in the rosette down to the bottom of the plant.
Root succulents, also known as caudiciforms, are some of the strangest looking plants of all. A caudex is a great big swollen part of the very bottom of the stem where it joins the roots or a big swollen part of the roots itself.
These plants can look just like big boulders, with twining surface growth of very soft trailing stems and leaves. One of them (Testudinaria elephantipes) is called elephant’s foot, because it looks just like a big grey foot!
Sometimes you can see the swollen caudex sitting on the top of the soil. Due to their shape they are sometimes called “living sculptures”.
But sometimes, like Ceropegia woodii, they look very like an ordinary plant (a mesophyte), until you look under the soil surface and find the swollen root there for storing as much water as possible.
Plants produce flowers so that they can be pollinated by insects or birds. Flowers are often perfumed and they produce a sweet sugary liquid called nectar that insects and birds come to feed on. The insects come to collect the nectar and at the same time, although they don’t do it on purpose, they collect pollen from the flower’s male parts and take it to the female parts of the same or more usually another flower of the same species. This is called fertilization. A fertilized flower produces seeds which will start new plants.
The top of the flower stalk holds the flower and it is called the receptacle, which means a container. The rest of the flower is called the perianth and this is made up of male and female parts.
The male part of the flower is called a stamen.and it is made up of an anther, which is the pollen sac and a filament which is holds the anther up so that the pollen can be spread by birds insects or the wind to fertilize the female part of a plant.
The female part is called a pistil and you will find it in the middle of a flower. It is made up of a style which jojns the stigma at the top to the ovary at the bottom. The ovary contains eggs in little containers called ovules and if the egg is fertilized, the ovule turns into a seed.
The other parts of the flower are the sepals, which are like little green leaves that protect the bud until the flower opens, you will see these at the bottom and outside of the flower. All of the sepals are called a calyx. Petals are the colourful part of the flower, which is often perfumed so that the insects will want to come to the flower to gather the sweet nectar from nectar glands and collect pollen at the same time. All of the petals are called a corolla.
Cactus and succulent plants also have beautiful flowers. In a cactus the receptacle is often altered into a long tube.
Some people think that cacti don’t flower or that they only flower once every ten years. But most cacti and succulents will flower every year just like every other plant. Some of the little globular cacti (round cacti) like rebutias flower when they are really small, and can make a lovely spring bowl which will flower every year and make a big change from daffodils and hyacinths!
Some of the very tall columnar cacti need to be very big to flower, so they will only flower after a few years when they have grown over a metre tall.
Lots of the other succulents have interesting flowers. Echeveria flowers grow on a snake like structure called a scorpioid cyme.
Agaves grow a huge spire of flowers, which can reach up to the roof of a building, but unfortunately the huge plant dies as soon as it flowers, so this is another plant that does not flower unless it is very old. It is also known as the century plant because people used to think it only flowered every hundred years. Well it doesn’t take a hundred, but it does take a very long time and the plant will be up to 2 metres or more across!
Aloe flowers look just like red hot poker flowers you see in English gardens because the families are related.
Cacti and other succulents like to grow in a frost-free sunny place and they like to be kept dry in winter. You can keep them on a sunny windowsill, in a porch or conservatory or in a greenhouse.
This means that you can keep cacti in places where other plants would not be happy because they would shrivel up in the bright light. Cacti and other succulents will love a bright spot and they won’t need watering often either so these are very easy plants for you to look after yourself. The leafier succulents will grow in positions that are not quite so bright.
You can put your plant in pots on their own or together in a big pot, but try to put the same types of plants together – so you could have a bowl of cacti, or a bowl of lithops or a bowl of leafy succulents. If you mix them all up together they don’t grow as well because some of the leafier succulents like a bit more shade and a bit more water than cacti and lithops, and they also grow faster so they will bully the smaller plants and take over their space.
You already know that your succulent plants like to have a nice bright position, but what should you plant them in?
Succulent plants will grow in any of the composts that you can buy from garden centres (and shops like B&Q and Homebase in the UK) but they do not like to have too much water. They are not like ordinary houseplants that like to be slightly damp all the time. They like to be completely dry before they have any more water given to them.
You can add grit or sharp sand (a coarse, open sand which helps with drainage), but you don’t have to do this. It is just to make sure that the water drains away quickly so that the plants don’t get too damp. You can also buy special compost, made for cacti, which already has lots of grit in it.
Succulent plants grow wild in habitats or places in the world where they do not have all the fungi and moulds that we have here. Our native plants are used to them but succulent plants aren’t so they can go rotten or catch fungal diseases if they are wet all the time.
In the late spring and summer…
…water your plants when the compost is completely dry. Give them a good soaking and then let them dry out again completely before you water them again. This means that you will be watering your plants about every 7 to 10 days.
Every two or three weeks the plants also like to be fed. This way they will stay healthy and grow more quickly and they will be more likely to flower, too. You can use a fertiliser that is made for tomatoes because although succulents don’t look like tomatoes they like the same plant food! Or you can also buy special food made specifically for cacti and succulents.
In the autumn…
…start to cut down the water you give to your plants from October, so that by November they are totally dry. Between November and March is their dormant period when your plants rest. This rest helps them to flower the following year. If they are kept somewhere that is frost free but cold, don’t give them any water at all. If they are warmer – on a house windowsill for example – then give them a TINY bit of water to stop them shrivelling. Using a hand sprayer and giving them a light spray is the best way.
In the early spring…
…start to water your plants again. Start off by giving them a little bit of water, and then let them dry out again. You will see that your plants begin to look fatter, which shows that they are taking up the water. You can gradually build up the amounts they are having until you are back to…
In the late spring and summer…
…water your plants when the compost is completely dry… and then start all over again!
Eventually your plants will out grow their pots and will need to go into bigger ones if they are to keep growing happily and healthily. This means that they will need re-potting. Only do this in the growing season (spring and summer) because the plants might not react well if you move them in the autumn and winter when they are trying to be dormant.
Do this somewhere where it doesn’t matter if you make a bit of mess! Try to work on a table and get all your things together first. You will need the plants that are too big for their pots, some bigger pots (the next size up) and compost.
Get a bigger pot that will give your plant a bit more room without swamping it. This is the next size up from the one your plant is in. Half fill the pot with compost.
Give the pot with the plant in a sharp knock against the side of your work table to loosen it.
If your plant isn’t spiky turn the pot over and let the plant fall into your hand upside down, then pop it the right way up again into the new pot. Fill the pot up all around the plant with new compost up to about 1cm from the rim, so that you will be able to water the plant without the water spilling out.
If your plant is spiky then roll up a strip of newspaper into a long thin strip and put this around the plant as a handle. You can hold the edges together and lift the plant out of its pot without getting prickled. Drop the plant gently the new pot, and carefully add compost around the plant in the pot. You can also do this with a pair of kitchen tongs.
After potting up your plants, leave them for two or three days before watering them again so that they can settle in.