Usually this species forms dense groups of 20-40 cm tall rosettes (stemless or short-stemmed), which face outwards and often almost lie on their sides.
Each rosette has about 20 incurved leaves of about 30×7 cm; they are green to greyish green and firm in texture, with rough, sandpaper-like surfaces and margins with reddish-brown teeth.
The nodding, 4 cm long flowers are dull red to pale scarlet (rarely yellow), appear in December and are arranged in branched inflorescences up to 60 cm tall.
This beautiful species occurs on arid, sandy flats from the Richtersveld to Loeriesfontein, Calvinia and Klawer. Unfortunately it does not thrive outside its natural habitat.
Falcata means sickle-shaped, referring either to the curved flower stalk or to the leaves curving inwards (both possibilities are mentioned in literature).
All pictures were taken just south of Vanrhynsdorp late July 2017, apart from the last one, which shows a plant growing near the office buildings of the Tanquana Nat. Park.
The epithet leucothrix means “with white hairs” and refers to the conspicuous glandular hairs on the leaves which make the species easily recognisable.
It is a small, sparsely branched shrub, usually no more than 5-8 cm tall, with a main stem that is 1.5-6.5 cm thick (normally partly underground), with peeling bark.
The leaves are arranged at the stem tips; they are narrow, 1-7.5 cm long and 0.2-1.5 cm wide, the upper surface grooved. They are dry at flowering time.
The inflorescence is up to 35 cm long and bears 0.6-1 cm long tubular flowers (yellowish-green to pale yellow with pink to almost white lobes) in October to February.
One usually comes across the species under bushes on south-facing (shaded) rocky slopes throughout the Little Karoo.
Without flowers, this species is similar to two others (P. horombense and P. rosulatum), but they are easy to tell apart when flowering.
The plants are widely distributed in the central highlands of Madagascar, where they are often abundant in thick layers of black humus on inselberg slopes and rocks at altitudes between 200 and 1750 m.
They grow into shrubs of 10 to 50 cm tall and 1.5-2 m in diameter.
The branches are densely covered with pairs of spines and each has 3-5 deciduous leaves (to 10 cm long and 5 cm wide).
The inflorescences are 25 to 40 cm tall, with up to 30 flowers at the same time.
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
These large shrubs or small trees are up to 6 m tall, usually much-branched from near the base. The young branches are softly woody and have big leaves (to 6 x 8 cm), with 3-5 triangular lobes.
The unisexual flowers are creamy-white with red stripes near the base and produce large green capsules. Oil from the seeds was traditionally used for lighting but nowadays also for manufacturing glycerine, soap and biofuel.
The species is widespread in dry bush and forest in southwestern Madagascar, especially on limestone, and is the only Jatropha that occurs naturally on the island.
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.
