(Lithops): Structure and Function

Lithops are a group of small succulents that are native to Namibia and South Africa. The name "Lithops" comes from Greek and means "stone like", referring to their aptitude for camouflage  within their rocky desert habitat. Lithops are cone shaped with a divide down the middle. They can be though of as two fists side by side with their palms touching. They are much smaller than fists though, with most only reaching 1 to 3 inches wide and growing half an inch to an inch above the soil in the wild. Most of their mass lies within the soil so as to be better camouflaged and conserve water. In captivity, these traits are less crucial so many lithops are grown sticking further out of the soil. The two "fists" are actually leaves which connect directly to a taproot. This means that lithops do not have a true stem. Older lithops have multiple pairs of leaves that are connected to the same root and grow in a cluster. Lithops are able to live a surprisingly long time of 40 to 50 years. Lithops are angiosperms, meaning that they flower. Many species flower in the later fall and early winter, although some flower in the spring or early summer. The flower emerges from the fissure between the two leaves can be many different colors including yellow, white, and pink, depending on species. Most lithops species have to be at least 3 years old before flowering. 

(Lithops): Evolutionary Relationships

Because lithops usually grow mostly underground, they have adapted a means to photosynthesize that is different than many plants. Rather than just allowing the tops of their leaves to photosynthesize as much a possible, the top of lithops' leaves are window light to allow light in. This makes light travel through a larger amount of plant tissue to reach the photosynthesizing cells at midday. This protects the plant from the extremely high intensity light while allowing more diffused, softer intensity light to reach the photosynthesizing tissues. At the end of day, light has to pass through less clear tissue to reach chlorophyll, allowing it to keep more of its already lessened intensity. A diagram of this is shown below. Lithops's evolutionary history is a large mystery with very few details known about its phylogenic tree. 

(Lithops): Ecology

They often live in places with 20" of rain or less per year. Some species live in places with as little as 4" of rain per year. Due to this lack of water, their root are very large compared to their size. Their roots can reach 6 inches long while the tops are only 1 to 3 inches in across as described earlier. They are able to and like to grow in a medium consisting of just 1/5 organic matter, and the rest being mineral. They don't even need true dirt. Lithops are not toxic to humans, pets, and many other animals. There are some mentions of African children eating them to hydrate themselves. Because of their non-toxicity, their camouflage is even more important to avoid being eaten. To help blend in with their surroundings, many species have markings and patterns on their tops to breakup their shape visually. Lithops are becoming a popular houseplant, but are known for being particularly difficult to keep alive. Many of these plants die from over-watering or watering during the wrong season which will rot the plant. iNaturalist has many observations of lithops. Many of these observations are of plants that are much less showy in color and pattern than their house plant counterparts.