Depending on the time of the year, plants belonging to the genus Mitropyllum develop completely different leaf pairs.
During the resting period, the pairs consist of two leaves pressed together in an upright position. This cone-shaped entity is covered in a paper-thin skin, the remains of the preceding leaf pair.
At the beginning of the vegetation period, the leaf pairs start to swell, and the pressure causes the skin to rip apart. After a while, the old leaves are in a more or less horizontal position and in between them the early stage of the mitre-shaped leaves can be seen. In the next phase, the latter start showing their final shape. The original leaves now only bear the remains of the covering membrane. The mitre is still very thin but will fill out considerably in the weeks to come. Gradually the old leaf pair will shrivel and ultimately wither to dry remains. The mitre-shaped leaf has now attained the shape which the genus name refers to.
This phenomenon whereby plants possess leaves of more than one shape or size is called heterophylly.
In Mitrophyllum and related genera this means that the leaves of the hot resting period are smaller than those present at the height of the cool growing season, resulting in less loss of water.
M. grande is a shrublet with a compact centre and short-shoots from which erect long-shoots develop.
It has green to yellowish-green leaves, which in the first pairs are 6-12 x 1.5-3 mm, with tongue-shaped to triangular free parts; the second-pairs are fused for 4/5, forming an oval body 2.5-10 cm long and 1.5-3.5 cm wide.
The thick and soft internodes are 1-1.5 cm in diam.
Yellow or white flowers appear in May-July (Oct.-Dec. in the northern hemisphere) and are up to 4.5 cm in diam.
Restricted to a small area in the Richtersveld, where it occurs on S to SE slopes with quartzitic stones.
In cultivation a strict summer rest should be respected.
The conspicuously dotted leaves are typical of this species, which is one of the smallest in the genus.
Over time the plants form low cushions up to 2.5 cm tall and 18 cm in diameter.
The leaves are of two types:
one pair forms a body of 2-5 mm long with 2 very short lobes, greyish-green with a purple hue. During the hot and dry resting period, this pair dries out and forms a dry sheath-like cover which protects the consecutive pair.
In this second pair, the leaves are almost free, 2-7 mm long and 2-3 mm wide and thick, triangular in cross-section, and pointed.
This phenomenon of two different types of leaf-growth is called heterophylly and it may be interesting to note that it is reflected in the name of this species (bi=two; forma=form, shape).
The plants have solitary purplish flowers (with or without a darker mid-stripe) on stalks 3-4 mm long.
According to the literature, they occur in shaly sandstone crevices in the Swellendam area. The first picture below was made about 20 km NW. of Montagu, the others about 15 km E. of Montagu. The last two ones show plants in late January (during the resting period), the other ones were taken in early September (during the growing season).
Barbata means bearded and it would be hard to come up with a more apt name for this interesting little gem which is not just beautiful, but also interesting in an ecological sense.
The pictures show how dramatically the appearance of the plants changes between late autumn and late spring. Please bear in mind that the plants occur in the southern hemisphere, and also that they only grow in the cooler and wetter months (autumn to early spring).
In nature, these plants are short-lived (1-2 years). They are so-called monocarpic, which means they die after flowering (new rosettes may be produced at the base).
Their leaves are variable in shape and about 1-2 cm long and 2-3.5 cm wide. They are fringed with relative long hairs (cilia), which may be up to 0.5 cm in length and are able to absorb dew. Over a hundred years ago, Marloth found out that plants of this species could absorb in this way more than 5 % of their weight per night.
Although the plants are stemless, they are up to 30 cm tall when in flower (September-November).
This subspecies is widespread from the Cederberg to the Little Karoo, always occurring in small numbers, as a rule in shade, under shrubs or on rocky slopes.
Ssp. broomii is only known from near Victoria West and mainly differs in the much shorter cilia (less than 1 mm).
Mid May (Late autumn)End of JulyEarly AugustEarly AugustLate SeptemberEarly OctoberEnd of October (Late Spring)
With summer approaching, the rosettes have closed to minimize transpiration. The cover of long hairs also acts as insulation against strong light and desiccating winds.
The outer skin may also be enclosed in a layer of cork or wax; this will make the surfaces of leaves and stems practically waterproof. It will also prevent the heating of the leaf, which lowers temperature and thereby the rate of transpiration from the leaf. This is found in many succulents and other xerophytes. Plants can raise the amount of wax when the temperatures are very high or the relative humidity is very low.
The white wax layer of Adromischus leucophyllus also reflects lightThe peculiar Othonna herrei is completely covered in wax
Usually the plant’s water is bound in substances that do not release water easily, so that even if the plant is damaged, the inside stays moist for a long time. (Tanquana prismatica).
The bladder cells we met before, do more than storing water. When the plants start suffering from drought stress, the cells collapse and thereby obstruct the passage of air to the breathing pores so that water loss is reduced. The picture shows a flower bud of Mesembryanthemum guerichianum.
A rather uncommon adaptation is that in which young parts of the plant are sticky, so that when sand is blown unto them, the grains form a protective coating. This occurs in Psammophora and Arenifera, two genera which derive their names from this phenomenon (sand bearer in Greek, resp. Latin) but also in about 50 other species, both succulent and non-succulent. The pictures show Crassula columnaris v. prolifera (above) and Psammophora modesta (below).
EVASION Unlike animals, plants can not move away when the going gets tough. They have to endure the bad times in situ and that is one of the reasons why we find many succulents in shade: under shrubs, in rock crevices etc.
There are many advantages in sheltering under another plant: less sun, less wind, lower temperatures, difficult to be found and reached by browsing animals. In these 2 cases (Haworthia arachnoidea above and Gasteria disticha below), only the inflorescences give away the fact that there is a succulent hiding in these bushes — and you have to look very carefully to find the rest of the plant itself.
Hiding underground also minimises water loss from the surface of the plant. Experiments have shown that in plants such as this Lithops julii ssp. fulleri, the loss is about a fifth lower in plants that are buried than in exposed ones.
RECYCLING
Many members of the mesemb family, especially the dwarf ones, are able to recycle water from old leaves to new ones.
As the soil dries out, the older leaves are gradually sacrificed and their water content is translocated to and stored in the younger ones.
Within each old pair of leaves a new, but somewhat smaller one develops and stays dormant, while the dry remains of the old leaves form a protective layer of insulation for the new ones. When the rainy period starts again, the new pair bursts through the old skin, ready for action. It has been found that this adaptation enables plants to survive for about a year without any moisture from outside.
Mesembryanthemum (Prenia) sladenianum is a well known example of this phenomenon
Below are pictures ofAntimima pumila in the growing season (early September), at the end of October and in high summer 3 months later, showing different stages of water recycling.
To stay alive, living organisms must be able to maintain a healthy water balance. In other words, over a certain period at least as much water must enter them as leave them.
Adaptations for collecting and storing water obviously are not very useful without suitable means to conserve the water as well.
As Gordon Rowley in his “Illustrated Encyclopedia of Succulents” (1978) put it so nicely: ”A storeroom for water is of no use if it lacks a door to prevent the contents from escaping”.
Although some water is lost through the surfaces of leaves and stems, most of the loss is caused by transpiration through the breathing pores, so the key to the water balance is reduction of transpiration.
Transpiration rates are also influenced by factors such as temperature, humidity, presence and intensity of sunlight, precipitation, wind, land slope etc.
On hot days, the soil surface may reach a temperature of 75 ° C, whereas a few centimeters higher up it may be up to 40 ° C cooler. In between, there is a layer of still air.
Similar conditions exist above plant surfaces and evidently they have a strong influence on transpiration. The layer is easily thinned or even destroyed by wind.
Many succulents have their leaves or stems arranged in such a way that pockets of still air are formed.
Often, these outgrowths also provide a certain shade and reflect or scatter sunlight hitting the plants.
The stems or leaves may also be covered in a mantle of spines, hairs etc. This coat produces a layer of more or less still air and thereby reduces transpiration; it also gives a certain amount of shade, which helps to diminish exposure to strong radiation – especially when the cover is light in colour.
Spines serve the same purpose as hairs. Usually they are modified branches, leaves or parts of leaves (but in the case of Euphorbia they are often hardened flower stalks and in Monsonia/Sarcocaulon they are hardened leaf stalks).
HAIRS Crassula mesembryanthemoides
Pelargonium barklyi
In Huernia pillansii the hairs are in fact very elongated tubercles
SPINES Monsonia crassicaulis
Euphorbia eustacei
STIPULES
The name of this plant (Avonia papyracea) refers to the white papery scales which hide the tiny green leaves almost completely. The scales are actually stipules (outgrowths at the bases of leafstalks); in this case they are much bigger than the actual leaves and protect these against sun and wind. At the same time the stipules are able to trap water.
In Pelargonium hystrix the stipules look rather different
As we have seen, the ability to store water is the main characteristic of succulent plants. The most important aspect of the stored water, is how much of it can be withdrawn from the storage tissue and become available to the rest of the plant. This fraction is called the utilizable water and is very different from one succulent to the other.
We usually talk about leaf, stem or root succulents, depending on where most of the water is stored. But because many plants have succulent tissues in more than one type of organ, the distinctions are not always clear.
In many stem succulents, ribs and tubercles help the stems expand and contract as their water content changes. They also assist in directing water from the stems to the roots and in shading different parts of the stem whenever the sun is shining.
Ribs: Stapelia grandiflora (1) and Quaqua pillansii (2)Tubercles: Senecio stapeliiformis (1) and Euph. clandestina (2)
In leaf succulents we sometimes see similar anatomic features, which make changes in volume possible in the storage tissues and thus in the leaves. As the water volume of these leaves decreases, this will lead to folds in them at fixed places, so that they become shorter and flatter.
Curio (Senecio) abbreviatus in the rainy (1) and in the dry season (2). Picture 1 courtesy George HattinghIt has been found that in plants such as this Aloe pearsonii, the leaves may lose up to 60 % of their water and still be able to recover
The globular shape we see in many succulents is optimal for water storage, because it combines maximal volume with minimal surface area.
Euph. globosa
Some plants have an epidermis with so-called idioblasts (extremely enlarged and swollen bladder cells). These may supply up to over half of the total water storing capacity of the leaf.
When the plants start suffering from drought stress, the cells collapse. This obstructs the passage of air to and from the stomata, so that water loss is reduced.
This external storage is most common in the Aizoaceae.
Drosanthemum hispidum (1) and Mesembryanthemum barklyi (2)
Of all the problems succulents face in nature, the main one is the scarcity of water.
In most dry areas the rainfall is unpredictable, with long periods of drought. For at least part of the year, this goes hand in hand with a low humidity (in daytime often lower than 20%), which in combination with high temperatures leads to high evapotranspiration.
Even when there is rain, only part of the water becomes available to plants. The more water comes down in one go, the more runs off or otherwise disappears, especially in rocky and sandy soils.
When water becomes available, succulents must be able to collect it as quickly and thoroughly as possible and they have developed a number of adaptations for that purpose.
Most of them have a shallow rooting system, mainly at a depth between 5 and 15 cm and often extending 10-20 m away from the plant.
A disadvantage of such a root system is that the topsoil may become very hot (70° C or higher) but it allows the plants to absorb even small quantities of rain, dew or mist. The roots often end underneath stones, where they form dense mats. Stones condense dew and mist and collect the water at their base and in crevices; they also protect the roots against drying out.
Shortly before dawn, plants too often reach such low temperatures that a substantial amount of dew accumulates on their surface. (Argyroderma delaetii and Cephalophyllum curtophyllum)
In the dry season many succulents lose their fine roots, but even a little bit of moisture will quickly cause the growth of very fine so-called rain rootlets, which will then absorb nearly all the available moisture.
When enough water becomes available, succulents may take up so much that they literally burst.
A plant such as this Augea capensis may increase its weight several times after rain
This plant of-Fenestraria rhopalophylla is just a few cm across but has roots which may cover up to two m2. Fenestraria occurs in the mist zone on the coast of northern South Africa and southern Namibia, where the sea mist is the main source of water.
Because dew and mist are often more reliable than rainfall, they are important sources of moisture for other plants as well. These plants of Aloe ferox grow not far inland from the south coast in South Africa and regularly receive mist rolling in from the sea.The large leaves of this Aloe microstigma cool down sufficiently at night to collect dew. In many places dew may be the main or even only source of water for months.Mesembryanthemum ( Prenia) sladenianum has spoon-shaped leaves that are perfect for condensing dew and mist
At night, protuberances such as spine tips, hairs, papillae, bladder cells, thorns and spines will become cooler than the rest of the plants and the surrounding air, so that dew condenses at them and is channelled to the roots.
In certain cases, the water can be absorbed directly by these appendages.
Crassula sericea var. velutina uses inflatable epidermis cells
The white scales of this Avonia papyracea are so-called stipules, outgrowths at the base of the leaves. In this case they are much bigger than the leaves themselves and protect these against sun and wind. At the same time these stipules are able to trap water.Crassula barbata
Trichodiadema marlothii Back in 1908, Rudolf Marloth in his famous book Das Kapland, reported the following on plants of this genus: “As soon as one puts a drop of water on a hair at the tip of a shrivelled leaf, the cells suck it up and in a short while the leaf is plump again”.
The term hydathodes is normally used for structures that are in control of guttation (loss of water in the form of drops from the margins of leaves).
The hydathodes that are found in nearly all species of the genus Crassula perform the opposite function: taking up condensed water and atmospheric water vapour. They are arranged in one or two rows along the margin and/or they are distributed over the surface of the leaf. and are often surrounded by trichomes (hair-like growths) which are supposed to assist in trapping water.
Crass. sladeniana
Crass. nudicaulis var. platyphylla
In some plants, only the young leaves have hairs, whereas the older ones are smooth. The first of the two pictures above shows a young plant of Tylecodon paniculatus. In the second picture a mature plant of the same species is accompanied by a plant of T. wallichii (on the left). These species sometimes hybridise in the wild.With their many thin stems, broom-like Euphorbias such as this E. burmannii do not look very well equipped for dry conditions. It seems however that they are rather effective in collecting condensed dew and mist and channelling this moisture to their roots.The same applies to other thin stemmed plants such as this Crassula muscosa v. obtusifolia
EXCESS OF ENERGY
In general, organisms can only survive if the quantities of water and energy entering are at least as big as those leaving them.
Succulent plants usually receive too much energy (solar radiation, wind) and too little water, which makes their balancing act even more complicated.
Radiation In many arid areas the light intensity is usually high – often 2-2.5 times as much as in more temperate climates – and because there are few clouds, there are also more hours of sunshine.
When the radiation becomes too high it may damage the plants’ chlorophyll or overheat their tissues.
Succulents have developed a variety of means to reduce these dangers (upright stems or leaves, hairy or wax-covered skin, covers of spines etc.). Many columnar succulents bend in the direction of the sun and in leaf succulents the new and vulnerable leaves often grow upright at first, becoming more horizontal over time.
One of the many different forms of Cotyledon orbiculata. The whitish wax covering reduces absorption of light by about 10%.More or less the same will apply to the greyish leaves of Crassula perfoliata ssp.minor. The fact that these leaves are upright also helps in diminishing the effects of high radiation.
The tiny upright columns of Anacampseros filamentosa.In young plants of giant aloes like this Aloidendron pillansii, the leaves are more or less upright.In older plants of the same species, the leaves are about horizontal. The upright position of the young leaves most probably reduces the effect of solar radiation, which helps young plants survive.The hairy leaves of this Pelargonium caroli-henrici do not only reflect the light, but also create a layer of still air (which diminishes transpiration) and are able to collect dew.
Another way to avoid the dangers of high radiation is hiding under shrubs, in rock crevices etc. This also helps against strong winds and browsing animals.
Haworthia scabra
Anacampseros telephiastrumCrassula sericea
Temperatures As a result of the large amounts of radiation, temperatures are often high during the day. In many cases there are great differences between day and night temperatures (often more than 50°C) as well as a great variation over the year.
The temperatures of soil surface may even reach 70°C.
Because the roots of succulents are usually near the surface, this is bound to cause problems. Under rocks it is much cooler, so many roots are found there. No wonder many succulents seek shelter near or in between rocks.
Cheiridopsis namaquensisDorstenia foetida
Whereas reduction in size is a good strategy for coping with lack of water, miniature succulents (by definition growing near the soil surface) will be severely affected by these high temperatures. The same applies to young seedlings. Plants in general may get rid of excessive heat by means of transpiration, convection or long-wave emission, but often these options are not available to succulents:
– using transpiration to lower body temperature would lead to unaffordable water losses
– their low surface area-to-volume ratio reduces the boundary layer where convection can take place as well as the area from which long-wave radiation may be emitted.
Wind
Wind is usually present and often hot and strong. The continuous replacement of air around the plants has a desiccating effect so that water loss can be extreme.
In many arid regions sandstorms are a regular occurrence, transporting not only dust and sand but often also small stones, damaging plants and remove hairs or wax cover through abrasion in the process. Seedlings are especially vulnerable in this respect.
Crassula columnaris ssp. prolifera with a protective cover of grains of sand.
PREDATORS
Predators: sometimes they eat you, sometimes they feed you. Othonna sedifolia surrounded by animal droppings.
In the case of this Gasteria disticha the advantages seem decidedly one-sidedEven a little browsing animal can do a lot of harm. (Tanquana prismatica)
Of the 10 spiny mesemb species Ruschia spinosa is probably the most widespread one. After the fruits have disappeared, the supporting bracts turn into sharp spines that protect the new leaves. (Other functions of the spines will be discussed later.)
Because of their juicy contents, succulents are attractive to herbivores, but many of them are unpalatable. This may be because they are very bitter (Aloes), contain a sour, salty sap (Augea capensis, Mesembryanthemum crystallinum, M. guerichianum) or a milky latex (Euphorbia ssp.), or even because they are poisonous (Cotyledon, Euphorbia, Sarcostemma, Tylecodon).
The harsh environment where succulents usually grow causes many problems and the interactions between problems and solutions are often quite complicated.
I realise that studying a diagram like the one below is not everyone’s cup of tea, but if you are interested in the way succulents survive in nature, spending a little time on it may be well worth it.
Legend abrasion: damage caused by strong winds transporting sand and small stones.
evapotranspiration: the process by which water is transferred from the earth to the atmosphere by evaporation from soil and water surfaces and by transpiration from plants. evasion: avoiding an undesirable situation by hiding under bushes, in rock crevices etc. life forms the shape and structure of a plant as a result from adaptation to its environment. predator: an organism that obtains food by eating other animals or plants.
The one thing that sets succulents apart from all other plants is the ability to store water to allow the plants to remain active even in periods when no water is available.
Although this does not make them immune to extreme lack of water, it of course helps to alleviate problems caused by temporary droughts.
Succulence is often described as a structural phenomenon, something you can see and feel, so that plants are called ‘‘succulent’’ when the leaves, stems and/or roots have a swollen appearance.
We can also look at succulence from a functional perspective, wondering how it affects the way a plant functions and survives in its habitat.
As these posts are not intended for professional botanists but for amateur plant aficionados, they are mainly concerned with the visible adaptations succulents have developed.
When looking at a collection of succulent plants it is easy to see that “some of them are more succulent than others”.
It is sometimes less easy, or even nearly impossible, to say whether a certain plant should be called a succulent or not. In other words, succulence is a continuum and there is no clear cut-off point between succulent and non-succulent plants.
Not allsucculents arecreated equal
So when we talk about ‘‘succulents’’, we should bear in mind that this is in fact shorthand for more correct but also slightly complicated phrases, such as ‘‘plants with notable succulence’’ or ‘‘very succulent plants’’.
There are many different kinds of succulent features, which may be combined in several ways and water may be stored in more than one organ. For these reasons it is very difficult to give a comprehensive (as well as comprehensible) definition of what succulents are.
The following definition is easy to understand: “A succulent is a plant that stores water in its tissues as a mechanism to survive periods of drought in the growing phase.” List of southern African succulent plants, G. F. Smith e.a., 1997.
The two definitions below are more comprehensive: “A succulent is a plant possessing at least one succulent tissue. A succulent tissue is a living tissue that, besides possible other tasks, serves and guarantees an at least temporary storage of utilizable water, which makes the plant temporarily independent from external water supply, when soil water conditions have deteriorated such that the root is no longer able to provide the necessary water from the soil” Life Stategies of succulents in deserts, D. J. Von Willert e.a., 1992.
“Succulence can be defined as follows: = storage of utilizable water in living tissues in one or several plant parts in such a way to allow the plant to be temporarily independent from external water supply but to retain at least some physiological activity.” Living under temporarily arid conditions succulence as an adaptive strategy, Eggli U. and R.Nyffeler, Bradleya 27/2009, pages 13-36.
Succulence is a tried and tested solution for plants living in dry environments, but it is not the only possible strategy for surviving there. In the wild, succulents are often growing together with plants that use very different adaptations to stay alive.
The ability to store water has developed independently in around 50 different plant families. For many plants it is apparently important to have some degree of succulence, as it occurs in 4-5 % of higher plants. Estimates vary from about 8000 to plus or minus 13.000 -out of approximately 260.000 species (depending on what you call a succulent and how you define a species).
As it is of little use to store something that you cannot keep safe, succulents have also developed a variety of means to conserve the stored water.
It is this combination of storing and conserving water that causes the peculiar appearances of these plants.
When you follow this link, you will be taken to a gallery giving you some idea of the enormous diversity in size, shape and structure of (mainly South African) succulents.
