To see this species in the wild, you have to go to the far north of Madagascar. The plants grow there abundantly on limestone on the Montagne des Français and neighbouring hills near Antsiranana. The old -but still often used- name for this town is Diego Suarez (hence the specific epithet).
The plants are solitary, with or without a short stem, with leaves up to 60 cm long and 10 cm wide.
They produce inflorescences 60-80 cm tall, with 4-12 branches. The flowers have a length of about 2.8 cm and are covered with very short, soft hairs.
Comparing the first three pictures (taken in the wild) to the fourth (taken in cultivation), it may be hard to believe that they represent the same species.
The plants occur in quartz gravel on hillocks on the southwestern Knersvlakte.
They are geophytes, with a tuber up to 1.5 cm tall and 2 cm wide and 8-14 leaves, which are about 5 mm wide and die back at flowering.
The inflorescence is to 15 cm tall, with about 10 flowers, appearing in late spring / early summer (October-November).
Subspecies namaquensis differs from its sibling by having no more than two leaves, one of which is usually inconspicuous.
The inflorescence is shorter (5-10 cm tall) and always single and the filaments have a double tuft of hairs.
These plants occur only in the Northern Cape, from Springbok to the Richtersveld in gravelly places.
One often comes across the name of the species as Bulbine mesembryanthemoides, but because Haworth used the spelling mesembryanthoides in his original description, this has to be accepted as correct.
Subspecies mesembryanthoides has a small underground tuber and usually 1-2
leaves (rarely up to 4). The leaves are cylindrical, 1-2.5 cm tall and up to 2 cm in diameter. As soon as the dry summer period arrives, they start to whither.
Flowers appear in spring and summer (Augustus-November), usually after the leaves have retracted. Each plant may have 1-3 inflorescences up to 20 cm tall.
This subspecies occurs widespread from the Knersvlakte in southern Namaqualand to Graaff-Reinet in the Eastern Cape on rocky slopes and flats.
If there were a list of favourite types of succulents, I’m sure the so-called window plants would rank very high. Most of these plants belong to the Aizoaceae (Mesembs), but they are also found in Bulbine and Haworthia.
When one looks up information on window leaves, one gets the impression they are all built on the same principle: the surface of the leaf tip lacks chlorophyll, the central parenchyma* reaches up to the epidermis and as a result of this combination, the leaf tip looks and acts like a window.
In a few cases however, the windows are formed differently.
It has taken me quite a while to find a place where this is described in a comprehensive as well as comprehensible way.
In Cactus & Succulent Journal (US) vol. 16, 1974, Werner Rauh published an article called Window-leaved succulents. He starts his explanation with a description of Haworthia obtusa (=cooperi) var. pilifera and uses more or less the following wording:
“The numerous fleshy leaves are nearly hidden in the substratum and we can only see the transparent, glass-like leaf tips, ending in a long hair. The transparence of the leaf tips is caused by a lack of chloroplasts**. We find the assimilation parenchyma only in the lower two thirds of the blades, but these leaf parts are not accessible to the light. The consequence of this anatomical structure is that light, necessary for assimilation, can reach the assimilation parenchyma only by passing the transparent windows. But the leaves of H. pilifera are not in the morphological sense true window leaves.”
He then moves on to Haworthia obtusa (=cooperi) var. dielsiana, saying:
” … the most remarkable feature is the behaviour of the leaves in the course of their development: young leaves are of the same shape as those of H. pilifera, but becoming older, the upper third of the lamina, which exceeds the soil surface, dies off, so that only the water parenchyma, covered by the shrunken epidermis is to be seen. The lower parts of the leaves with the assimilation parenchyma are hidden in the ground; sunlight can reach it only by passing through the water parenchyma.”
This is the same type of window formation we find in Bulbine mesembryanthoides. In Rauh’s words:
“Becoming older, the upper parts of the leaves die off, as in Haw. obtusa var. dielsiana and the result is the formation of a big window with a plane surface. The assimilating parenchyma is completely hidden in the substratum.”
We know that strong sunlight destroys the chlorophyll, which is essential for the plant’s metabolism. Window-leaved plants are hidden in the ground (at least in the hot and dry season) and sunlight can reach the assimilation tissue only through the windows, passing through the water parenchyma. This filtering process protects the plants against very strong light.
In experiments carried out with Fenestraria, it was found that the light is reduced so much that the chloroplasts will not be damaged, but stays strong enough to allow sufficient assimilation and production of organic substance.
* parenchyma is the relatively undifferentiated tissue that makes up the bulk of many plant organs and is often used for storing of water or food.
** chloroplasts are the tiny parts within plant cells that contains chlorophyll.