SEMPERVIVOPHILIA : Variability and Polymorphism

WHY and HOW are Houseleeks such variable plants? If there is an essential topic to understand the genus Sempervivum, it is certainly this one. Indeed, ALL nomenclatural and taxonomical problems linked to this genus ensue more or less directly from it.

1. Generalities

The genus Sempervivum, as defined elsewhere, can be considered as homogeneous and well circumscribed. Unfortunately, it is not the same for most of the "species" (and numerous infraspecific taxa) that constitute it. If it is relatively easy to connect the members of this genus to some collective species (named then by the terms "aggregates", "complexes", or "grex") rather well defined, it is often very difficult to go beyond, even with a good knowledge and a good experience of these plants. Indeed, most taxa of the genus Sempervivum are not only morphologically close to each other, but are characterized by a high variability expressed by a wide polymorphism and a high reactivity to the conditions of the environment that makes their determination to be as difficult as the diagnosis and descriptions attempting to define them are fuzzy and intricate.

Term and notion of "collective " species are fought by some botanists, who consider these are either species in the same way as other species in the usual sense, or these are more or less artificial groups having to be divided in several species or infraspecific taxa. However, how can the "nebulas" constituted by some of these "species" such as S. hirtum, S. tectorum, or S. marmoreum be interpreted without this notion. In fact, the notion of collective species seems a reality, even though it antagonizes the assumptions of the classic nomenclature. Seen like a dynamic evolutionary process, a collective species is not any longer a species in the usual sense but it is not yet a group of related distinct species nor a set of infraspecific taxa able to be clearly individualized. A collective species, it is the intermediate state between these states. Only a too Cartesian vision, or even a fixist one, can deny this evidence.

The consequence of this variability of the genus Sempervivum was the creation, at some time, of a real "sausage-nomenclature" of the kind "superfine slices", complex and fussy. It is very little usable, because of its hypertrophy and especially its absence of structuring and global vision of the genus. But especially, this former nomenclature remains very little representative of the reality, because of multiple partitionings of imprecise levels, partitionings little reminiscent of the narrow links and relations between the taxa, in spite of the obvious multiplicity and diversity of these.

A realization just as unsatisfactory and indigestible will be the "ravioli-nomenclature", that followed the "sausage-nomenclature" (after having preceded it initially besides...), regrouping under a few vast common taxonomical envelopes a mire of ingredients of origin as diverse as inhomogeneous and binding the whole by a sauce as artificial as doubtful (cf. History of nomenclature).

What therefore about the famous polymorphism and the variability of the members of this genus mentionned consensually by all the authors, and which caused and will probably cause again much ink to flow?

This variability presents itself under several very distinct aspects about their mechanisms, but intricate and often inseparable in the reality of facts. It should not therefore be forgotten that attempting to analyze individually their modes, as here, is certainly a didactic necessity but shows a somewhat artificial side.

Factors and mechanisms of variation stated below are not of course particular only to the Houseleeks, and integrate the larger framework of variation in superior plants, or even in living organisms in general. However, the genus Sempervivum shows them in a manner so exhaustive, at a level so elevated and demonstrative, and especially in a manner so associated, that this genus is about this subject a real "text-book case":

2. Variation according to the conditions of the environment

The environment is to consider as the sum as much as the interference of the climatic, edaphic, and biotic conditions. Variations observed between the populations of the same species in relation to the different environments, and induced by these, can be explained by two fundamentally different mechanisms:

Important note: all noticed variation between some populations of different environments is not necessarily (or only) linked to the conditions of these environments, as one will see it below.

2.1. Interaction of a genotype and its environment

Most sempervivums shows, in their natural habitat, an astonishing morphological plasticity in answer to the different conditions of these environments and to the variations of these. One observes therefore what is called a formation of accommodations, i.e. some merely phenotypic variations linked to various conditions of altitude, sun, temperature, pluviometry, substratum, etc. and to the various periodisms linked to these external factors. This capacity of genotypes to modulate their expression thus and to induce very differential accommodations (a capacity that one designates by phenotypic plasticity) is observed with a rather high frequency in numerous mountain plants and seaside plants with sufficiently varied and extended biotopes, but it is here particularly marked. One can speak therefore for Houseleeks about a strong accommodative reactivity.

Thus, it can be very difficult to make in situ the connection between two different samples of Sempervivum from different places, whereas once introduced in uniform cultivation, these samples, initially very unlike, will present a perfectly identical aspect (and often very different from the first two aspects in situ!).

Besides, two different accommodations of different species but growing in the same conditions, can sometimes show between them such likeness as that sometimes leads to confuse them, often much more looking alike between them than two accommodations of a single species growing in contrasted environments would do. However, once placed in uniform cultivation, in conditions different from those of their original environments, the characters of the first ones will often diverge considerably whereas on the contrary the characters of the second ones will come closer (such a phenomenon of convergence of the accommodations can, for example, sometimes be noted in some shared areas of S. calcareum and S. tectorum).

2.2. Selection of ecotypes

An ecotype is a genotypic variation, due to the natural selective pressure associated with a determined environment. Contrary to an accommodation, that is a variation of the only expression of the genotype, therefore the genotype remains stable, an ecotype is a variation at the level of the genotype itself.

The selective pressure of the environment, linked to the various parameters of this environment, will increase at the level of the genomes that are submitted to it the frequency of some favorable alleles in these given conditions and will decrease that of the unfavorable alleles, this with the passing of the sexual generations. This mechanism is as well valid at the level of the lineages of individuals as at the level of the global populations. The so-selected alleles can either preexist in the genes pool of the initial taxon, or can appear secondarily in this pool by aleatory mutation.

This phenomenon of selection of ecotypes is generally of little importance for what concerns the variability in the usual sense that is given to this term. Indeed, "variability" is often identified, wrongly, with morphological variability alone. The selection of ecotypes entails mainly an appearance of variants (or in the end of vicariants) of which the adaptation to the environment is essentially of physiological nature, therefore of more difficult and less obvious study at first sight.

Thus, as in a lot of other genera, and maybe more than in a lot of other ones, a nomenclature of the genus Sempervivum that would be based only on the morphology would be only an incomplete or even approximate one because of its fickleness. The physiological criteria are as much discriminative as the pilosity or the colour of a petal... The morphological criteria can even be considered as just the tip of the "genotype-iceberg". But the great difficulty consists in apprehending and qualifying these physiological criteria as taxonomical criteria. The latter are just as much determinants as the former. The aim of this note is not to suggest pulverising the nomenclature a little more by a pleiad of useless sub-races of sub-sub-varieties, but merely to show in evidence the important notion of "physiological taxon" and that some characters can be as real, constant, and meaningful, if not more, than three hairs here or there. However, the general difficulty to qualify them and to quantify them without the help of techniques often evolved, complex, multidisciplinary, and especially... as long and as expensive (as biochemical analyses, enzymatic studies, chromatography, immuno-electrophoresis, controlled hydric and nutrient balances, etc.), makes them disregarded, which is regrettable, or even ignored, what is unforgivable. However, the methods of the phytosociological reports and analyses allow a presumptive detection of ecotypes without recourse to these complex techniques (except for the unavoidable statistical calculations...), and find there probably their main and finest justification. To say that in a simpler manner, if a characteristic plant of a given association is found in another type of association, it is likely that there are in fact two differentiated ecotypes of this plant, even though the two morphological aspects are strictly identical.

Attention, it must be clear that the murderous remarks made elsewhere in this document about Phytosociology, tackle its nomenclatural approach, therefore its formal concept, but not the principle itself.

The counterpart of this misjudgment of the merely physiological characters is, alas, the overconsideration of the morphological criteria as criteria of classification. Let's recognize however that these must remain the basis of the Linnean nomenclature, this in a simple convenient mind, but the basis must not constitute the whole.

2.3. Notes about the notion of adaptation

It has been seen that an ecotype is defined by its adaptation to the features of the environment that shelters it. This adaptation of an ecotype is necessarily the reflection and the consequence of the appearance at some point of some selective advantage(s) in some individuals of an initial general population submitted to the conditions of a precise, limited and homogeneous environment. The advantage factor may also preexist in a potential manner in some individuals, behaving then like an indifferent factor (or even sometimes as a more or less pejorative one) and becoming an effective advantage only after a modification of the environment. The survival of these individuals and/or the transmission of their characters to the progeny will be facilitated then by this advantage, which defines the notion itself of selective advantage. The facilitation of the maintaining and/or the reproduction of the individuals carrying some avantage(s) necessarily occuring at the detriment of the non-carriers, this will entail in the end a variation, therefore an evolution, of the average range of the genotypes in this population.

However, one must not to be mistaken about this notion of adaptation of an organism in its environment. Its meaning being often badly understood, the considerations linked to it risk therefore to be interpreted badly. Indeed, the more an organism (in the sense of a population) is adapted to a definite environment the more it is dependent on it. That means that:

A local adaptive advantage is not therefore necessarily synonymous of a real advantage in the sense of an increase of probability of maintaining and transmission of the concerned genotypes in the time. A short-term adaptive advantage does not necessarily result in a long-term evolutionary advantage for the concerned organism. From a terminological point of view, one should not therefore to interpret the terms "evolved" and "adapted" as the expression of an any scale of values in relation to "primitive", "initial", "original", etc. but as the simple observation (or suspicion) of processes inherent to natural evolution of the living organisms. These terms constitute therefore more a scale of time (i.e the "evolved" is necessarily of more recent appearance than the "primitive") than a scale of values. Evolution being an universal and non orientated biologic phenomenon, it is essential not to lose sight of that:

Natural evolution of living beings does not have a finality, it only has some consequences.

...and every contrary affirmation would drag us out of the framework of Science to enter the one of philosophical interpretation... At the scale of Times and Life, every narrow adaptation of an organism to a precise environment, as effective it can seem, increases paradoxically the risk of disappearance in the end of this organism or the risk that this adaptation ends in an evolutionary cul-de-sac. Thus, the obvious vigour of the tree of Life conceals in fact a lot of dead or sterile branches...

In plants, one can sometimes suspect to be in the presence of ominous consequences of a too narrow adaptation when one notes for a species or a simple form that...

...this misleading phenomenon is not exceptional and is the indicator of a big reactivity and potential fragility to various factors of modifications of the environment of which the anthropic factor is not the least...

This digression is not superfluous for a good understanding of the variability in the genus Sempervivum and the interpretation of its multiple geographical forms, of which some are very local.

3. Interindividual variation

The variations observed between some individuals growing in identical conditions inside the same population, and which persist at the time of introducing these individuals in uniform cultivation, express the fact that, in Sempervivum, most species and infraspecific taxa (therefore genomic types) are intrinsically variable, the notion of 'Type' therefore having to be considered only as a necessary (?) nomenclatural convention.

In situ, populations of Houseleeks often present as many morphological types as there are tufts present! Nevertheless, these interindividual variations, although genotypic ones since the environment conditions are homogeneous for all individuals, are not fixed hereditarily, and demonstrate only a high level of heterozygosis of the genotypes for a great number of characters. At each sexual generation , the range of the genic palette is mixed again and divided by the meiosis then reconstituted by the fertilization.

The elements of interindividual variation the most frequently noted in Sempervivum concern mainly the general size, shape of the leaves, shade and distribution of the pigmentation, and to a less marked manner the more or less globular character of the rosettes (by secondary growth of the leaves, not by reaction to the drought), pilosity and glandulous character of the latter, abundance and length of the stolons. In short, practically all vegetative features can be more or less implied. Characters of inflorescence are much less, as is generally usual. However, variations of size and ramification of it are neither rare nor limited, but in this case it's difficult to appreciate what is a matter for the domain of phenotypic accomodation, of the intrinsic genotypic variation of a single taxon, or of a real specific or infraspecific criterion (numerous "species" have been described formerly on this single criterion). The floral criterion which is the most distinct indication of interindividual variation is the number of floral divisions (this one can vary however sometimes slightly in the same individual...), that, in a given species, is indeed more often a range of values than an absolute criterion. Besides, floral and inflorescencial criterias are globally very homogeneous in the genus (except the hiatus between Sempervivum proper and Jovibarba), and are therefore insufficiently discriminative for the establishment of a precise systematics, the subtle semantic nuances by former botanists about the aspect of the hypogynous scales remaining of very delicate appreciation and use...

4. Perannual individual variation

Characters of shape, colouration, pilosity of the rosette are strongly linked to the progress of the vegetative cycle, without it can be established because of that some general rules about this variation through the year. Thus, numerous species show a very marked summer pigmentation whereas others will show their maximum colouration at the time of the winter rest, or only at the spring start of vegetation, to show then a rather uniform green at the heart of summer. Leave pilosity can also present an important perannual variation which will make that an individual of a species considered as pilous could be perfectly glabrous at some times of the year and conversely.

A plant observed at one time can thus be totally different from the conventional aspect of its description, an aspect that it will show maybe of a characteristic manner a few weeks later.

These deep changes of aspect during cycle can therefore easily lead to confusion, or even some risky comparisons because of the momentary impossibility to recognize and to differentiate some taxa between each other at some periods of the year, whereas a little later their aspects will diverge completely. It is therefore important, for identification, that all descriptive or diagnostic morphological characters refer to one precise period, that, classically and by default, is that of flowering, although it is not necessarily always the most discriminative one.

This important perannual individual variation also has as a consequence the frequent impossibility of immediate identification of a study sample, the observation of the evolution of its aspect at the time often being an indispensable element for this. One sees therefore that field identification of some Houseleeks is not always easy and can require, before any conclusion, their cultivation and their observation during a complete vegetative cycle, and if possible continued up to flowering. The identification of a sample can require therefore one or even several years before being able to give out a conclusion... that is often only a hypothesis!

5. Interannual individual variation

It is only a particular aspect and a direct consequence of the interaction of a genotype and its environment, mentionned above (cf.).

Indeed, some small climatic variations at local level, unavoidable and aleatory, can entail some considerable variations of the aspect of these plants from one year to the other one. Thus, prolonged rainfall or drought entail an obvious morphological adaptation in Sempervivum in situ. It is the same for any variation, even minimal, of the cultivation conditions: meteorological ones, nutritional ones, place, etc. The morphological variation affects then the general shape as well as the pilosity or the colourations. Maybe it would also be necessary to include in the elements of interannual variation, although it is more questionable, the production of stolons (and that out of context of flowering), because these are curiously, in some species or individuals, some years as rare as they were abundant the previous year, or the inverse, all conditions seeming otherwise identical.

Taking photographs at the same date, during several consecutive years, of the same tuft of Houseleeks in cultivation, is generally sufficient to demonstrate the reality and the importance of the interannual variation in these plants. One will have then often the impression of observing two different plants!

5.1. Notes about anisophylly

This very particular morphological adaptation can also be observed as well in situ as in cultivation and is of very variable intensity according to the years for the same individual.

En culture, on l'observe certaines années surtout lors du passage brutal d'une période pluvieuse à une période ensoleillée, essentiellement en début d'été. On constate dans ce cas, chez beaucoup d'espèces, l'apparition d'une importante asymétrie des rosettes liée à un développement asymétrique des feuilles, c.à.d. une anisophyllie. Les feuilles externes supérieures s'allongent en s'érigeant à la verticale, les feuilles inférieures restent inchangées ou s'érigent elles aussi à la verticale mais sans s'allonger. Une interprétation finaliste verrait là soit une sorte de "rideau pare-soleil" des feuilles inférieures protégeant le cœur des rosettes ou, au contraire, une optimisation des surfaces de photosynthèse des feuilles supérieures. En fait, il semble que ce phénomène soit loin d'être univoque , et d'interprétation difficile, tant son apparition semble (faussement) aléatoire suivant les plantes et les années. L'un des facteurs en cause, en plus du facteur climatique qui vient d'être signalé, semble être le stress minéral, des apports nutritifs abondants semblant en effet le minimiser (mais cela resterait à prouver par une expérimentation rigoureuse).

In cultivation, one observes it some years mainly at the time of a quick transition from a rainy period to a sunny one, especially at the beginning of summer. One notes in this case, in many species, an appearance of an important asymmetry of the rosettes linked with an asymmetric growth of the leaves, i.e. an anisophylly. The upper external leaves lengthen while erecting themselves at the vertical, the lower leaves remain unaltered or erect themselves at the vertical but without lengthening. A finalist interpretation would see there either a kind of "sunshade curtain" by the lower leaves protecting the rosettes heart or, on the contrary, an optimization of the surfaces of photosynthesis of the upper leaves. In fact, it seems that this phenomenon is far from being univocal, and of difficult interpretation, so its appearance seems (falsely) aleatory according to plants and years. One of the factors in question, in addition to the climatic factor that has just been mentionned, seems to be the mineral stress, somes abundant nutrient supplies seem indeed to minimize it (but that would remain to be proven by a rigorous experimentation).

This phenomenon isn't noted only in cultivation but also in situ in some individuals. In this case also it seems to have rather a relation with climatic conditions because one meets distinctly more numerous anysophyllic samples during the summers with variable weather than during those with well established good weather (this is only a simple personal impression that requires further confirmation).

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