Here is another article about plants with truly startling and unusual foliage, a short series I intend to add to from time to time. This article therefore follows Five Plants With Weird Foliage, Four Other Plants With Weird Foliage and Plants With Weird Foliage: Perfoliation. Just click on the links if you ever want to re-read them.
How Leaves Function… Normally!
Although there are nearly 400,000 species of plants on our planet, most have leaves with exactly the same structure. First, there is a dark green upper surface. Its color comes from chloroplasts, the green cells that convert sunlight into energy and are located just below the leaf’s upper surface. The underside of the leaf, though, has few chloroplasts and is therefore a paler green. This organization is very logical, because the plant’s goal is to capture a maximum of solar energy and the sun is located above the plant, not below. Even the way most leaves are held on the plant, that is, horizontally, is designed so they can absorb all the solar energy possible.
When the Sun Is Too Intense
So much for a typical leaf! But some plants, especially those of arid climate plants, face a rather unusual situation. The sun where they grow is so intense it can burn the leaves. Also, they can’t possibly absorb all the energy it produces. Most plants living under arid conditions have had to find some way of protecting themselves from the sun’s excesses.
These plants have different strategies to get around an overly intense sun. Sometimes the leaves are covered with wax or hair that reflects rather than absorbs light, sometimes the plant sacrifices its leaves entirely and photosynthesizes through its green stems (cacti are good examples of this) and sometimes the plant gives up entirely, losing its leaves and retreating into dormancy, often underground, during the hot season. But of all the adaptations to an overbearing sun, window plants have come up with the most fascinating adaptation.
Most window plants pull themselves down into the ground during the summer, leaving only the tip of their leaves exposed. And this exposed part is not green, although it may look that way at first, but rather translucent, like a window. Thus, the intense and burning light penetrates through the tip of the leaf, but is then diffused by the gelatinous translucent sap inside and redirected to the chloroplasts which are located inside the leaf, near the outer walls, and therefore literally underground. It’s all rather like a skylight. The transparent leaf tip of window plants is rarely sharply pointed, as that could lead to water loss. It is rather truncated or rounded, as that reduces the surface exposed to drying winds.
This ingenious adaptation, which botanists call a leaf window, an epidermal window or fenestration, has evolved not once, but several times in different families. The best-known window plants are the living stones of the Aizoaceae family, a group which, broadly viewed, can be said to include Lithops, Fenestraria, Frithia, Ophthalmophyllum, Conophytum and several other genera. However, there are window plants in other families, including the Asphodelaceae, Asteraceae and Piperaceae families.
Curiously, the vast majority of window plants come from the same region: the deserts of southern Africa. What is not clear, though, is why. What is so special about the conditions in this region that stimulates plants to develop—independently!—a window rather than or in addition to other methods of surviving drought used by plants in other desert climates, such as succulence, summer dormancy, reduction of stomata and others? I’m sure someday botanists will find an explanation.
The Mother of All Window Plants
The most windowlike of the window plants is undoubtedly Fenestraria rhopalophylla, in the Aizoaceae family. In fact, it is often simply called “window plant” in English (baby toes is the other common name) … and that’s also the meaning of its botanical name Fenestraria. This plant forms a rosette of upright pale gray-green tubular leaves, each capped with a rounded and completely translucent tip: it looks like it was wearing a contact lens!
In nature, only the translucent tip is visible, the rest of the plant remaining buried. When grown as a houseplant, we like to expose more of the leaf, partly to highlight the plant’s curious form, but mostly because it’s hard to imitate the intense drought and dry heat of its native country in our homes: if we bury the leaves the way they grow in the wild and the soil around them remains the slightest bit moist, the poor plant tends to rot.
Frithia pulchra, also in the Aizoaceae, is very similar, with the same tubular leaves and rosette growth, but this time the ends appear truncated rather than rounded. And the flowers are pink rather than white or yellow (the case with Fenestraria). This plant too is commonly called window plant or baby toes.
The plants most often referred to as living stones are in the genus Lithops (Aizoaceae) and all have windows, but they are not as apparent as those of Fenestraria or Frithia because the window is marbled with paler opaque patches that mimic the coloration of the neighboring rocks.
Each living stone (and there are dozens of species) consists of two succulent half-moon leaves pressed against each other, plus a few roots. The leaves can be green, gray or even reddish.
Among other living stone genera that include species with windows are Conophytum and Ophthalmophyllum.
The genus Haworthia is closely related to the better-known genus to Aloe (both belong to the Asphodelaceae family) and indeed, most species look much like small aloes, with succulent leaves, a rosette growth habit and sharply pointed leaves.
Species with windows, such H. cymbiformis and H. retusa, sometimes called cathedral window haworthias, are different. They have leaves with a more rounded, translucent tip and in nature, live essentially underground with only that part of the leaf exposed.
The most extreme window haworthia is H. truncata, sometimes called horse’s teeth. The end of each leaf is “truncate,” as per the species epithet truncata, meaning it looks like it has been cut off … with a saw. The leaf tip appears dark green, but is, in fact, translucent. This species usually grows in a fan shape rather than the more typical rosette common to haworthias. It’s a distinctly odd-looking plant!
Another genus of the family Asphodelaceae, Bulbine, also produces a few window plants (notably B. haworthioides and B. mesembryanthemoides). Their succulent leaves form an entirely flattened, ground-hugging rosette marked with translucent patches. Curiously, many other bulbines have fairly ordinary succulent leaves, much like an aloe, and others have deciduous grasslike leaves and underground bulbs or tubers. They spend the dry season safely underground, fully dormant.
String of pearls or rosary plant (Senecio rowleyanus), a popular houseplant in the Asteraceae (sunflower) family, is also a window plant, but its window is rather discreet. Its small leaves, almost as round as a pearl with just a small pointed tip, are medium green … but this is not the part of the leaf that carries out photosynthesis. If you look closely, you’ll see that each leaf has what looks like a darker green ray like a cat’s eye, but which is, in fact, transparent. It’s through this slit that the light penetrates the leaf and reaches the photosynthetic cells on its inner periphery.
This window often goes unnoticed and many people grow this fairly common succulent without realizing how truly extraordinary it is. The window is best noticed when the plant is backlit by sunlight and it then appears yellowish green and distinctly more translucent.
Unlike other window plants seen so far, string of pearls does not grow half-buried, but usually completely exposed. The rounded leaves are borne on long, thin, creeping or trailing stems and it grows as a groundcover in the wilds of southern Africa, its stems rooting where they touch soil. As a houseplant, it’s most often grown in hanging baskets as a trailing plant.
The chain of bananas (S. radicans) is similar in habit, but, as the name suggests, its more elongated leaves look somewhat like bananas. String of beads (S. herreianus) fits in between the two. Its succulent leaves are somewhat rounded, yet more pointed than S. rowleyanus. Both have the same kind of very narrow slit-like window.
A Leaf in Prayer
The window plants we’ve seen so far all evolved independently in southern Africa, but there is one major exception.
In the vast genus Peperomia of the family Piperaceae, with over 1500 species distributed throughout the tropics, there is a handful of species of window plants, all from Peru and Ecuador. The logic behind their fenestration is not so clear as with the African succulents, because these peperomias don’t live in a desert environment, but rather in tropical forests, often as epiphytes. But they are succulents, with thick leaves that store water, something that can be useful to an epiphytic plant, given it has no soil to protect its roots from dehydration and is constantly exposed to drying winds.
What is fascinating with the best known of these window plants, the prayer peperomia (P. dolabriformis, whose specific name means “shaped like a doloire”, a kind of axe) is that it seems to have been caught midway through its evolution, as if it weren’t quite finished. Just looking at the strange leaf, you can easily see that what was originally an ordinary elliptical and flat leaf has folded upward and inward, like a praying hand (the origin of the common name prayer peperomia) as if to protect its upper surface. What was originally the paler green back of the leaf is now borne upright with a slightly depressed window now separating the two halves. The window looks dark green, but is actually transparent and sunlight can travel through it to the chloroplasts lining the inside of the leaf.
There are other peperomias with a similar habit. This group includes P. nivalis and the very interesting P. graveolens, where the outside of the leaf is red and thus contrasts strikingly with the green window in the center.
P. ferreyrae, with narrow, pointed, scimitar-like succulent leaves, seems to have evolved further than the others, because you no longer notice the effect of a leaf folded in half. Its windows are present, but quite discreet.
Finally, the most bizarre of all the window peperomias is undoubtedly the columnar peperomia (P. columella), a short, upright plant whose small, stubby, very succulent leaves look like they were chopped off at the tip.
The window plants are truly fascinating … and many of them make attractive, easy-to-grow, thought-provoking houseplants. Place a window plant in front of your window today!