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LITHOPS EVOLUTION

The evolution of Lithops plants resulted probably in a transition from a stage characterized by green and spread leaves to a stage characterized by attached and coloured leaves. This is easily understood if you observe most of ice plants from less desert areas (for example, Carpobotus edulis [1] ). Molecular analysis showed that three species of Lithops with  greenish leaves ( L. divergens , L. helmutii and L. naureeniae ) split earlier from the group, then we are entitled to assume that the appearance of a primitive Lithops could reasonably be compared to that of these three species. According to our hypothesis , L. pseudotruncatella should be considered, with respect to this feature, the more specialized species of the group. In all Lithops species, the leaf cleavage is completed after the replacement of the cotyledons by the first pair of true leaves , but in some species ( such as L. aucampiae ) , it is slow to form and, practically only in L. pseudotruncatella, we need to wait 3 or 4 years to observe its full extent throughout the “head”.

 

Only in Lithops pseudotruncatella the two old leaves do not separate completely in adult plants during the shrivelling phase, differently from what occurs in other species. On the contrary, we attend the formation of a odd "sleeve" that wraps around the plant (also called the hymen ): the old leaves are separated only in the central area, but they remain substantially joined at the sides and then eventually drying up on the new growing leaves (when they are dry, the sleeve is easily removed by wind or weather) .

                                  

This peculiar way of leaves replacement is very similar to that observed in the hyper- specialized Genus Conophytums. Conophytums shares the same desert habitats of Lithops but, unlike them, they may present pairs of leaves so closely welded to form a single unit: the appearance is that of a single rounded leaf with a hole (or a short slit) at the center.

 

The evolutionary explanation of the large variability in leaf markings is far from obvious in Lithops and would probably be a waste of time, in the state of the current knowledge, wondering why, for example, some species exhibit markings in the form of dots (L. terricolor, for example) and the other in the form of lines. What is shared to almost all types of markings, however, is their ability to make the plant less recognizable on the background where it is located. That background, it should be remembered, in no case consists of smooth pebbles, but instead it is composed of stones which are characterized by very different shapes and which are frequently angular-sharped. It is clear that in such environmental circumstances, a plant with rounded and symmetrical leaves, with a very  uniform texture, would certainly be detected more easily.


 
Lithops dorotheae

 
 

Lithops bromfieldii var. insularis 
in habitat (from: www.cactuspro.com, Marc Mougin)



It is interesting to note, in this regard, as certain species positioned outside of the genus Lithops seem to have adopted a strategy which is different and in some ways opposite to that of our beloved living stones. Lapidaria margaretae, for example, [ 2] , has an uniform colour (very light gray), but the edges of the leaves are angular-sharped, so that to create an effect of light and shadow that allow it to blend in an environment composed of stones with the same features.

Except some cryptic species (e.g. , Conophytums pellucidum var . terricolor ), the Conophytums [3] are often characterized by a green color that does not allow them to camouflage well in the desert environment in which they grow. However, it should be noted that during the dry season - the season during which the succulent plants are more vulnerable to predators, given the lack of other sources of vegetables - they present a protective (grey and heavily ridged) sleeve as a remainder of the previous season. The suspicion is that precisely such an habit help to hide the plant, otherwise very visible in its environment.

 

The subject of the classification of Lithops plants, both at a subspecific and a superspecific range, is far from be a settled issue but we will not treat it in detail in this place, limiting ourselves to follow the subdivisions suggested by Cole. It must be said that recent DNA analysis have only partially confirmed the phenotypic-based botanical observations:  they have led also to some quite unexpected results. The taxonomic status of Dinteranthus vanzylii is paradigmatic. This taxon has been the subject of constant movements to and from the Gender Lithops, now including it, now excluding it. When it seemed that systemtics had found an agreement about that (for his exclusion from the genus Lithops), here is a new DNA analysis forcing them, for the umpteenth time, to re-evaluate the issue. The sore point here seems to be, in fact, the extreme proximity of L. vallis - mariae to the genus Dinteranthus and, conversely , the extreme closeness of D. vanzylii to the genus Lithops: such a problem could be solved by ‘clusterizing’ Lithops and Dinteranthus in the same genus .

 

 

 
flowering Conophytum schlecteri


 
Lithops naureeniae


 
Shriveling in L. pseudotruncatella


 

Conophytum obcordellum at the beginning of the vegetative season



 

Lithops bromfieldii var. insularis 
in habitat 
(from: www.cactuspro.com, Marc Mougin)



 
Lapidaria margaretae (from wikipedia)


 

Dinteranthus vanzylii