Night Breathers: CAM Photosynthesis


Ill.: &, montage:

Do you remember how photosynthesis works from your biology class in high school? If not, here’s a quick revision.

In their green cells (chloroplasts), plants use the energy from sunlight, carbon dioxide (CO2) captured from the air and water absorbed from their roots to produce glucose they can later use for their growth and share, via their sap, with all their parts. Photosynthesis occurs during the day, while the sun shines. So, plants breathe in CO2 during the day through pores in the leaves called stomata and do all their photosynthesizing. They then carry out respiration at night, using the stored energy for growth and giving off oxygen and water vapor. Story closed.

Or is it?

Actually, some plants do their breathing at night, absorbing CO2 in the dark. Then they essentially “hold their breath” all day long, keeping their stomata closed. The CO2 they stored (in the form of malic acid) is then released for photosynthesis during the day, in the presence of sunlight. It would be the equivalent of a human being holding its breath all day and only breathing in the dark.

This remarkable aberration has been known for over 200 years, although it was only named in 1940. It’s called CAM: crassulacean acid metabolism or CAM photosynthesis. It was first discovered in plants from the crassula family, whence the name crassulacean.

Designed for Drought

CAM plants keep their stomata closed during the day; other plants keep them open. Ill.:

CAM is essentially an adaptation to drought conditions. The typical green plant, with its stomata open during the day, releases about 97% of the water it absorbs into the air through evapotranspiration. Yes, all that moisture you so carefully applied through watering is lost to the air and contributes nothing to the plant’s growth. What a shocking waste! 

CAM plants are much more efficient at water use. Because their stomata are completely closed during the heat of the day and only open during cool night hours, they lose much moisture to transpiration. They can therefore get along with much less water. 

Desert and arid-climate plants are therefore often CAM plants, but so are epiphytic plants: those that grow on tree branches. Constantly exposed to moving, drying air, they need to hang on to their moisture to survive and CAM is a good way of doing so.

There is a price to pay for this, though: CAM photosynthesis is much less efficient than regular photosynthesis and plants that use it grow more slowly than normal plants. But at least it allows them to survive difficult conditions.

Convergent Evolution

All cactus and most succulents are CAM plants. Photo:

Although in some plant families, all the plants carry out CAM photosynthesis (the crassula family is one), CAM actually evolved independently many, many times in many different plant families. When plants grow under arid or epiphytic conditions, they develop different adaptations to that situation (a waxy coating, improved water storage capacity, more efficient roots, etc.) and many “learn” to do CAM photosynthesis. 

Some 7% of all the world’s plants carry out CAM photosynthesis, including plants in over 300 genera and 40 plant families. Some do so exclusively. Others use regular photosynthesis under humid conditions and switch to CAM during periods of drought.

Among plant families where CAM photosynthesis is the rule is, as mentioned, the crassula family, but also the cactus family, both of which contain only CAM species. Both the bromeliad family and the orchid family have over 50% CAM species, which is not surprising, since most are epiphytes. But you even find CAM plants in such unlikely families as ferns (usually epiphytic ones) and cucurbits (the squash family).

Underwater CAM

Even a few aquatic plants (here Sagittaria) carry on CAM photosynthesis. Photo:

Curiously, there are a few aquatic plants, like Sagittaria and Littorella, that have also adopted CAM. There’s obviously no problem about losing moisture through open stomata with these plants, but CO2 is less available in water (it diffuses 10,000 times more slowly than in air) and during the day, when most aquatic plants are photosynthesizing, competition for it is fierce. So, they switch to CAM photosynthesis to get a better share of a rare product.

Crassula acid metabolism: it’s not the photosynthesis they told you about in school, but some of the plants you grow probably use it.

50 Houseplants That Don’t Mind Dry Air


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Most houseplants just don’t do well in desert-dry air! Source:

Dry air is a major problem for houseplants in the winter… and indeed, any indoor plant (seedlings, cuttings, etc.). When the atmospheric humidity is less than 40%, certainly common enough in many homes, plants try hard to compensate by transpiring more heavily, that is, by releasing water to the air through their stomata (breathing pores). The drier the air, the more they transpire, and that can lead to their tissues losing water more rapidly than their roots can replace it. This can result in all sorts of symptoms of stress: wilting, flower buds turning brown, leaves curling under, brown leaf tips, even the death of the plant.

And if that weren’t enough, leaves stressed by dry air are also more subject to pest damage (red spider mites, whiteflies, thrips, etc.)

Some Plants Can Cope


Plants with thick, waxy leaves cope better with dry air than those with thin ones. Source:

That said, many plants, especially those native to arid climates or ones where they are exposed to long periods of drought, have developed ways of compensating for dry air. Cacti and succulents are usually very resistant to dry air and so are some epiphytic plants, like hoyas.

Some plants resist dry air by producing leaves with fewer stomata than normal, thus reducing water loss. Many have abandoned leaves altogether and breathe through their green stems (many cacti, for example). Others keep their stomata closed during the day, when the sun is hottest and water loss is greatest, breathing only a night. (This is called Crassulacean acid metabolism or CAM.) In other words, they essentially hold their breath 12 hours a day! Also, plants resistant to dry air often have extra-thick leaves or leaves coated with wax, powder or hair, all of which reduce evaporation.

Plants That Don’t Mind Dry Air

What follows are a few houseplants that don’t really mind it if the air in your home is on the dry side. Not that they will suffer if you increase the humidity to levels more acceptable to plants in general (most plants prefer a relative humidity of 50% or above) and that indeed is good for your health too, but if improving the atmospheric humidity something you just can’t do, at least these plants will pull through without a complaint!

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Aeonium arboreum ‘Atropurpureum’: one example of a plant that tolerates dry air. Source, Bernard Dupont, Wikimedia Commons

  1. Aeonium spp. (tree houseleek)
  2. Agave spp. (century plant)
  3. Aglaonema spp. (Chinese evergreen)
  4. Aloe spp. (aloe)
  5. Ananas comosus (pineapple plant)
  6. Aspidistra elatior (cast iron plant)
  7. Beaucarnea recurvata (ponytail palm)
  8. Cephalocereus senilis (old man cactus)
  9. Cereus peruvianus (Peruvian apple cactus)
  10. Ceropegia woodii (rosary vine)
  11. Clivia miniata (clivia)
  12. Crassula ovata (jade plant)
  13. Crassula spp. (crassula)
  14. Cryptanthus spp. (earth star)

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    The thick leaves of the dieffenbachia can generally cope quite well with drier air, but you can see just a bit of damage at the tip of this one. Source: Forest & Kim Starr, Wikimedia Commons

  15. Dieffenbachia spp. (dumbcane)
  16. Echeveria spp. (echeveria)
  17. Echinocactus grusonii (golden ball cactus)
  18. Epipremnum aureum (pothos, devil’s ivy)
  19. × Epicactus (orchid cactus)
  20. Euphorbia lactea (candelabra spurge)
  21. Euphorbia milii (crown of thorns)
  22. Euphorbia tirucalli (pencil cactus)
  23. Ficus elastica (rubber tree)
  24. Ficus lyrata (fiddle leaf fig)
  25. Gasteria spp. (ox tongue)
  26. Gymnocalycium mihanovichii friedrichii ‘Hibotan’ (red ball cactus)
  27. Haworthia spp. (zebra plant)
  28. Hippeastrum cvs (amaryllis)
  29. Hoya carnosa (wax plant)
  30. Kalanchoe (kalanchoe, panda plant)
  31. Ledebouria socialis (silver squill)

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    Few plants tolerate dry air as well as living stones (Lithops). Source: Dysmorodrepanis, Wikimedia Commons

  32. Lithops spp. (living stone)
  33. Mammillaria spp. (pincushion cactus)
  34. Opuntia spp. (bunny ears)
  35. Pachypodium lamerei (Madagascar palm)
  36. Pelargonium graveolens (rose-scented geranium)
  37. Pelargonium × hortorum (zonal pelargonium, zonal geranium)
  38. Peperomia obtusifolia, P. clusiifolia (baby rubber plant)
  39. Philodendron hederaceum oxycardium (heartleaf philodendron)
  40. Rhipsalis spp. (mistletoe cactus)
  41. Sansevieria trifasciata (snake plant)
  42. Schlumbergera (Christmas cactus)
  43. Sedum spp. (sedum, donkey’s tail)

    20171227F Senecio rowleyanus Forest & Kim Starr, WC.jpg

    The nearly round leaves of Senecio rowleyanus are designed to reduce evapotranspiration. Source: Forest & Kim Starr, flickr

  44. Senecio rowleyanus (string-of-pearls)
  45. Senecio serpens (blue chalksticks)
  46. Stapelia spp. (carrion flower)
  47. Streltizia reginae (bird of paradise)
  48. Syngonium spp. (arrowhead vine)
  49. Yucca elephantipes (spineless yucca)
  50. Zamioculcas zamiifolia (zeezee plant)20171227A

Succeeding With Succulents


20170322AStephen Boisvert

Succulents on display at a plant show. Photo: Stephen Boisvert, Wikimedia Commons

Succulent plants or simply succulents are fleshy plants adapted to survive in arid environments. Their main feature is their ability to store water in their leaves, stems or roots in the form of sap. In fact, that’s how they got their name: the term succulent comes from the Latin “succus”, meaning “sap”.

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The thick sap in this aloe leaf is a water reservoir typical of most succulents. Photo: Raul654, Wikimedia Commons

Cactus or succulent?


This Rebutia is a true cactus, belonging to the Cactacaeae. Note the tufts of white fuzz from which circles of spines arise: they are areoles, only produced by true cacti.

Although you commonly hear the term “cactus and succulents”, it’s actually pretty redundant. Cacti are succulents, belonging to one of the few plant families whose members are all succulents, the Cactaceae. So all the cacti are succulents… but not all succulents are cacti, as there are plenty of plants in other families.

If you didn’t quite get that, it’s like poodles and dogs: all poodles are dogs, but not all dogs are poodles.

Adaptive Evolution in Response to Drought

Succulence has evolved independently many times in nature in many different plant families. In fact, some 65 families of plants have at least a few succulent members, including such unlikely ones as the begonia family and the orchid family.

In response to a climate that is becoming increasingly dry, those plants that are best able to tolerate drought survive and multiply while less drought-tolerant plants are gradually eliminated. When this process is repeated generation after generation, the survival of the fittest starts to operate in a big way, leading to plants that are less and less like their ancestors and more and more able to deal with drought.

Obviously, there are many ways to survive drought. Some plants learn to go dormant when water is scarce, others reduce the size of their leaves (most of the water that plants absorb is lost by evapotranspiration through the open stomata of their leaves), others adopt an annual lifestyle and learn to grow, bloom and go do seed rapidly after a rain before dying, others retreat entirely underground during the dry season, etc. Storing water reserves above ground in stems or leaves is however the most visible way plants face an arid climate.

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Euphorbia obesa has abandoned the use of leaves and carries out all its photosynthesis through its very plump green stem.

Many succulent plants, including cacti, abandoned their leaves along the way. These plants learned to carry out photosynthesis using only the chlorophyll-rich green cells in their stems. Thus leaves, which generally lose more water than stems because they’re thinner and ave greater number of stomata, were eliminated. In the case of cacti, those leaves were converted into spines.

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This succulent (Echeveria ‘Perle Von Nürnberg’) is so covered in waxy bloom that it seems more violet blue than green. Photo: Leonora Enking, Flickr

Other plants, such as the numerous plants in the Crassulaceae (crassulas, kalanchoes, sedums, etc.) and in the Asphodelaceae (aloes, haworthias, gasterias, etc.) families kept their leaves, but modified them, covering them with a thick cuticle, reducing the number of stomata and turning them into water storage facilities. Their leaves are often covered with white waxy bloom or dense hairs, both of which help reduce water loss by reflecting the sun’s excessively intense rays and thus reduce evapotranspiration.

Relearning to Breath

The evolution of succulents has also led to a rather surprising type of photosynthesis, called Crassulacean acid metabolism or CAM. (In spite of the name, this technique is not limited to the Crassulaceae, but developed independently in drought-resistant plants from many different families.)

Most plants open their stomata during the day so as to absorb carbon dioxide (CO2) and use it directly in photosynthesis which only occurs in the presence of light. In order not to lose too much water to evapotranspiration, they close their stomata at night, when there is no photosynthesis going on.

CAM plants, on the other hand, delay their respiration, opening their stomata only at night when the air is cooler and more humid and thus evapotranspiration is greatly reduced. Therefore they carry out respiration at night, but not photosynthesis, since there is no light. To be able to both breathe and photosynthesize, they store the C02 they absorb at night as malic acid. When daylight comes, they release the CO2 again, inside the leaf, and are thus able to carry out photosynthesis in the presence of sunlight, even though their stomata are firmly closed.

Think of it this way: it’s as if CAM plants hold their breath all day long, even though they’re running marathon, only breathing at night when they stop for a rest! (Ain’t nature amazing!)

Caring for Succulents

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Opuntia phaeacantha, here growing in the wild in zone 3 (Alberta), is among the hardiest of cacti. Photo: Ken Eckert, Wikimedia Commons

There are succulents adapted to cold climates, especially sedums (Sedum spp.) and houseleeks (Sempervivum spp.), that can be grown outdoors year round in even the coldest climates. There are even hardy cacti (especially certain Opuntia species)! But most succulents are tropical or subtropical plants and when we grow them in colder climates, it’s as houseplants.


These very unfortunate succulents are growing in terrariums, where high humidity and lack of drainage (so-called “drainage layers” are of no help at all) means their lives will almost certainly be very brief.

Succulents are now very popular with the general public and are used all kinds of ways, even in environments where they don’t thrive, such as in shady interiors and in terrariums. Stylists* have discovered something very interesting about succulents: they grow slowly… and die even more slowly. Even when they are mistreated, they can often survive for months, even more than a year. Therefore stylists can use them in situations that will be lethal for the plants and still get away with it. After all, when a plant dies 8 months after you bought it, you’re not likely to realize the fault lies with the person who designed the planting!

*I like to make a distinction between the people I call stylists, who tastefully arrange plants (slow-dying plants like succulents and air plants are their bread and butter) in containers without any real knowledge of or interest in the needs of the plants they use, and real horticulturists, who do know how to grow plants and would never treat them inappropriately.


Succulents spray painted unnatural colors. This is, of course, extremely deterimental to the plant.

Some con artists go so far as to spray paint succulents in bright colors (blue, red, pink, orange, even silver or gold!) before selling them to make them more eye-catching. You may hear them claim that this in no way harms the plant, but that is obviously nonsense. A covering of paint reduces the succulent’s ability to carry out photosynthesis, yet photosynthesis is the basis of any plant’s survival. Vendors of painted plants rely on the ability of succulents to survive long-term abuse without dying, that’s all. Such a treatment is more horriculture than horticulture!

Some painted succulents manage to put out new (therefore unpainted) leaves and thus recuperate. If so, bravo for them! But it still doesn’t mean painting them wasn’t mistreating them.

I hope that your goal in growing succulents is not just to see them as temporary decorations to be tossed when they’re no longer attractive, but living plants that you can encourage to grow and thrive. Along that line, here are some tips on really making them happy:



A corner window with light coming from both sides out to be excellent for succulents, but if you want them to grow well, don’t just open the blinds, lift them!

For the vast majority of succulents, full sun, that is, a spot directly in front of a window with a southern exposure, is ideal. Still, most will adapt fairly readily to the more modest light found near an east or west window, as long as they receive a few hours of direct sunlight each day. Most suffer, however, when they are more than 3 or 4 feet (1 m) away from any window. In winter, especially, when weak sun and short days mean sunlight is extremely limited (at least in homes outside the tropics), they really will do best placed close to a south-facing window.


Extremely etiolated cactus, showing many months of insufficient light.

When a succulent lacks light, it can show its distress in various ways: stems that stretch for the light (etiolation), abnormally pale new growth, branches or leaves that droop instead of remaining upright or leaves that are smaller than they were at purchase. This is the plant trying to tell you that it’s suffering.

However, some succulents are not so communicative: they simply stop growing when light is insufficient, looking apparently healthy for months, then die all of a sudden, without warning. If yours puts on no growth at all, especially in summer, that’s not a good sign!


The red edges on the leaves of this jade plant (Crassula ovata) appear only in summer and only when it is receiving the really intense light it prefers.

One sign that the plant is receiving adequate lighting is when the foliage becomes edged in red during the summer months. This shows that they really get the lighting they prefer. This phenomenon is particularly evident in crassulas and echeverias.


Haworthia fasciata is one of the few succulents that does quite well in moderate light.

There are, however, a few succulents that tolerate some shade (but must still get some daily sunlight). Some haworthias (Haworthia spp.) and gasterias (Gasteria spp.), especially those with dark green leaves, are quite shade-resistant, as are most snake plants (Sansevieria spp.). If you have to place a succulent well back from a window or in front of a north-facing window, they make good choices. Note however that although these succulents “tolerate” low light, they are not true shade plants and actually prefer at least moderate light (the snake plant, for example, will survive in shade, but will only flower if placed in the sun).


Most succulents have a distinctly seasonal growth pattern. They grow in spring and summer and slip into dormancy (or near dormancy) in late autumn and winter. Therefore, you’ll need to water more often between March and October and less during the winter. Excess watering when they are nearly dormant can easily lead to rot.


Water succulents thoroughly and let drain.

When succulents grow in a pot with drainage holes, watering is easy. During the spring and summer, water them like any indoor plant, completely humidifying the root ball each time: just pour out excess water that drips into the saucer. Then, before watering again, feel the potting soil, sticking a finger right into it, to make sure it is dry. For most of these plants, watering once a week should be quite appropriate during the growing season.

During late fall and winter, it’s better to let the soil dry out even more thoroughly, to the point it nears bone dryness. It’s more difficult to judge the watering needs of succulents at that season, but at room temperatures, watering every two weeks is usually enough. And if you keep the temperature very cold in the winter (40 to 50˚F/5 to 10 ° C), many will only need watering every 2 months!

When succulents grow in containers with no drainage hole, not only pots, but terrariums or decorative displays with nothing at all you could call a pot, watering becomes much more complicated. Just a little too much water and rot sets in. Usually you have to rely on the ability of succulents to tolerate abuse and water them less than they would prefer. This may leave them in a perpetual state of water stress, but they’re tough plants and can live that way for months, even years, without their suffering becoming too apparent. Try to water such plants a few spoonfuls at a time, but only when their soil is very dry.


Most of the time, home temperatures are adequate for succulents: they tolerate both summer heat and the relative freshness of winter without complaint. Nor are they much bothered by air conditioning.

Most desert cacti and agaves, though, prefer a cold winter with widely-spaced waterings, at temperatures between 40 and 50˚F (5 to 10 ° C). In the case of desert cacti, especially, a cold winter helps to stimulate spring or summer bloom.

Air Humidity

Air humidity is not a factor of great importance to most succulents. The air in the type of arid climate to which they are native often goes from extremely dry during the day to very humid at night when temperatures drop, much more than it varies in the average home, where the ambiant humidity rarely tops 50%, so adapting to your home’s range of humidity levels is easy for them.

What succulents tolerate poorly is when the air is constantly moist, as in a terrarium (see Cactus Terrariums: Easy On the Eye But Sooo Hard to Manage). If you do grow them under constantly high humidity, be very, very care not to overwater.


Succulents are slow-growing plants that need little room for their roots and so can grow for years in the same pot. When you repot them, ideally in the spring, you can use a cactus potting soil or even an ordinary potting soil to which you can add some sand for extra weight if you want. No drainage layer is required.

Many people like to cover the soil in succulent pots with a layer of decorative stones. That can be quite attractive, but remember that you have to push the stones to one side and sink your finger into the real potting soil to judge whether the plant needs watering.


Fertilize sparingly, at no more than 1/8th of the recommended rate, and only from March to October. Any fertilizer will do: succulents aren’t picky. Actually, even if you never apply fertilizer, they’ll still grow very well!


There are many ways of multiplying succulents: stem cuttings, leaf cuttings, division, seeds, grafting, etc. Which method works on which plants is however variable: it’s best to check out the plant’s particulars on-line or in a book about succulents before proceeding.

Insects and Diseases

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Stem mealybugs on an Opuntia. Photo: Ann Verga, Flickr

The three most common insect pests succulents are stem mealybugs, root mealybugs and scale insects. All come in on infested plants, so isolate any new plants for 40 days and inspect them carefully before introducing them to your collection. They can be hard to eliminate and it may be necessary to cull the infested plant.

Root rot and stem rot, caused by a whole range of fungus species, are the most common diseases and are best prevented by not overwatering and by avoiding excessively humid air. Once they set in, about the only thing to do is to try and take cuttings of any uninfected plant part.

A Summer Outdoors

Cactus Plants Nature

Succulents love to spend their summer outdoors.

Most succulents appreciate spending the summer outdoors. Just don’t put them outdoors if they grow in a pot with no drainage hole. Since there is no possibility of evacuating excess water in such pots, even moderate rainfall can lead to fatal rot.

Even if your succulent was growing indoors in full sun on a south-facing window ledge, that doesn’t mean it’s ready for that same full sun outdoors. After all, glass filters out UV rays and it’s UV that burns plants, so its needs to acclimate to the change. Place it outdoors in shade for a few days, then in partial shade for a few days, then in a sunny spot protected from midday sun for a few days. Only after that will it be ready for full, unprotected sunlight.

There you go! A quick overview of succulents and how to grow them. Enjoy discovering the beauty and ease-of-care of these simple plants.20170322AStephen Boisvert