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
