Soumission en 2007 au "Belgian Journal of Botany" sur les modes de tallage :

Plan de cette page :
1- la soumission
2- le rapport unique fourni par la revue en 2010, après nombreuses réclamations
3- quelques commentaires abasourdis sur cette réponse unique, obtenue après autant de temps pour un manuscrit si court

The taxonomic dimension of

the two types of tillering in Vascular Plants.

Daniel Chicouène
Arbiotech,
Kerbeneuc, 22250 Lanrelas, France.
daniel.chicouene@orange.fr

The two types of tillering in Vascular Plants

Abstract. - The study bases itself on formal work in plant morphology to demonstrate the value of distinguishing between different modes of tillering (as proposed by Hackel) in variously related taxa. These two forms of vegetative stem branching are studied together in the Monocotyledonae (Juncaceae, Juncaginaceae, Alismataceae), the Dicotyledonae (Ranunculaceae, Compositeae) and the Pteridophyta. Two different traits are used to describe the types of tillering: sheath, pierced or not ; orientation of the prophyll and the tiller base. The discussion covers the scope and limits of the definitions analysed for taxa descriptions.

Key Words. - branching, intravaginal tillering, extravaginal tillering, leaf, prophyll, plant morphology.

Résumé. - La portée taxonomique des deux types de tallage dans les Plantes Vasculaires.

L'étude est basée sur un travail de morphologie végétale comparée pour mettre en évidence l'intérêt de distinguer les deux modes de tallage (comme proposés par Hackel) dans des taxons voisins variés. Ces deux formes de ramification des tiges végétatives sont étudiées en même temps dans des Monocotyledonae (Juncaceae, Juncaginaceae, Alismataceae), des Dicotyledonae (Ranunculaceae, Compositeae) et les Pteridophyta. Deux caractères différents sont retenus pour décrire les modes de tallage: gaine, percée ou non ; orientation de la préfeuille et de la base de la talle. La discussion porte sur l'étendue et sur les limites des définitions analysées pour les descriptions des taxons.

Mots clés. - ramification, tallage intravaginal, tallage extravaginal, feuille, préfeuille, morphologie végétale.

INTRODUCTION

The taxonomical value in distinguishing between the two types of lateral branching that can occur with vegetative shoots (or tillering) was first shown in the Festuca (Gramineae) by Hackel (1882). The definitions proposed by this author were quite brief, based on whether or not the young shoot traverses the parental leaf-sheath (i.e. grows extravaginally or intravaginally). Hackel’s definitions of tillering were taken up by Arber (1934) who developed them further by pointing out that Hackel’s “branching called intravaginal” should be taken as “branches appressed to the parental axis”. For extra-vaginal branching, Arber (loc.cit.) stated that this corresponded to branches growing out horizontally or obliquely ; however such a definition is only valid if parental shoots are orthotropic. In recent literature, the use of this criterion in Festuca has varied and is not always taken into account, (e.g. Markgraf-Dannenberg in Tutin & al. 1980 and Kerguelen & Plonka 1989). In contrast, Stace & al. (1992), consider it to be a reliable criterion in separating certain infra-genera taxa of Festuca. Between the two extremes characterising different taxa, mixed tillering is frequent in Festuca, and insufficiently taken into account in the literature concerning this genus (Chicouène 1999 a, 1999 b) or Festuca rubra agg. (Chicouène 2002).

Other authors have also attempted to use this idea when writing about different genera of Gramineae (e.g. the genus Poa by Edgar 1986 but not by Negritto & al. 2008). In Flora Europaea, the type of tillering is sometimes taken into account (e.g. for Poa by Edmonson in Tutin & al. 1980), but is not without faults (which were first pointed out by Chicouène 1996). It is, possibly, the sole non quantitative morphological trait to differentiate Anthoxanthum odoratum L. from A. aristatum Boiss. (Chicouène 1996). It follows that, in the identification of certain Gramineae taxa, it is important to know the type of tillering.

The distinction between both types of tillering has already been used for other Monocotyledonae families : Cyperaceae such as Carex (e.g. Fournier 1936) and Alismataceae (Chicouène 2004). The same criterion has also been used in Dicotyledonae (as Ranunculus by Chicouène 2005).

Hackel’s definitions (1882) apply to innovation shoots, in other words to parental sheaths in good condition. For perennials possessing just seasonal vegetation and forming only one order of branching in the year, the parental leaf’s sheath will have disappeared by the time the bud develops. Hackel’s definitions will, therefore, be no longer of use for such plants.

In the Gramineae and Cyperaceae, the definitions proposed by Chicouène (1998) have a wider bearing and use more precise criteria. These are based on the way in which the prophyll is orientated to the lower order stem: either the prophyll and the bottom of the tiller lie parallel to each other (i.e. intravaginal tillering), or they are perpendicular to the stem giving rise to the offshoot (i.e. extravaginal tillering). These characteristics have the advantage of being useful when the parental sheath is no longer present.

In the case of normal buds (i.e. non extra-axillary) these two factors are worth considering in defining the type of tillering for other vascular plant families should a sheath be visible. It involves understanding the scope of the value of these ideas for use in comparative plant morphology and in taxa description. It is this which we attempt here, using our own observations.

METHOD

The present study is mainly based on the lowland flora of Western continental Europe, and details our own field observations (for more than 1000 taxa). The methodology is based on external plant morphology, whilst the taxonomy follows that of Flora Europaea (Tutin & al. 1980).

FAMILIES WITH BOTH TYPES OF TILLERS

In the Table 1, the two traits studied (i.e. sheath pierced or not and orientation of prophyll and tiller base) can be used for most of the species listed. In the case of Ranunculus paludosus Poir., however, only the orientation of the prophyll and tiller base is of help in identification of types of tillering.

Amongst the Angiospermae dealt with here the families, in which both types of tillering occur, may be either Monocotyledonae or Dicotyledonae. Tillering can sometimes differ from species to species within the same genus (e.g. in Luzula and Ranunculus) and can, therefore, be a determining factor in identification. Amongst the Pteridophyta, intravaginal tillering is found in those Filicopsida with an orthotropic rosette such as Blechnum spicant (L.) Roth, extravaginal ramification is observable in the aerial growth of Sphenopsida (e.g. in Equisetum arvense L.).

Table 1. Taxa within which both types of tillering are found (Gramineae and Cyperaceae excepted).
Legend :
Characters used : "s" = sheath, pierced or not ; p = orientation of the prophyll and the tiller base ; (g) = sheath bad differentiated.

DISCUSSION & CONCLUSIONS

The two descriptor organs compared (the sheath and the prophyll) are interdependent, and are simply different ways in which the same underlying origin expresses itself. However, they do not have the same taxonomical value. The way in which the prophyll is orientated is dependent on the way the base of the growing shoot is directed. It can be a key factor in identifying Ranunculus paludosus Poir. Even where the sheath no longer exists, the orientation of the prophyll is a more widely useful character, requiring only a little more observational effort. The sheath being pierced or not obviously explains the etymology of intra- and extravaginal, whilst the sheath being pierced or not remains secondary to the orientation of the prophyll and the tiller base.

Nevertheless, the definitions based on the traits studied remain difficult to use when the sheath no longer exists and where the corresponding inter-nodes are tubers, e.g. as in Arrhenatherum elatius (L.) Beauv. ex J. & C. Presl subsp. bulbosum (Willd.) Schübler & Martens.

In classifying the tillers in Festuca, Auquier (1973) took the architecture of the first leaves into account, noting that many basal leaves of intravaginal tillers have a blade as long as the leaves that form afterwards, while basal leaves of extravaginal tillers have a blade shorter than posterior leaves. However, within Monocotyledonae, reserve situation occurs in Brachypodium sylvaticum (Hudson) Beauv. (which has mixed tillering) and Luzula sylvatica (Huds.) Gaudin (with intravaginal tillering) can possess intravaginal tillers with many leaves lacking a blade. The type of tillering could be compared to burial of the tillering zone, since the bladeless leaves are often subterranean. However, it is an aspect which was not included in the present study. Our own observations suggest that the prophyll architecture studied by P. Bugnon (1921) included only those plants with intravaginal tillering.

The observations retained in this study are those corresponding to both extreme types of tiller. In defining possible intermediaries between both types of tillering, the orientation of the prophyll, with respect to the tiller base, has the advantage in that, without being strictly quantitative, can be estimated. Nonetheless, it can be thought of as a quantitative variable, with the sheath, pierced or not, remaining a specifically qualitative observation. A possible way of giving the latter a quantitative dimension would be to measure the height at which the sheath is pierced. This may at the base of the sheath if the tiller comes out more or less at a right-angle, or higher if it comes out at an angle. Such observations require the sheath to be in perfect condition. Such an approach would be a step towards perfecting comparison of species and studying plant variation.

Quantifying each descriptor enables extreme and intermediary types of tillering to be identified. Such ideas particularly apply to mixed tillering which has been observed in numerous Gramineae since Hackel. For taxa in this group, the whole of their cauline architecture may need to be revisited, a fact already emphasised by Chicouene (2002). However, such work would then enable the taxonomic value of tillering to be properly evaluated or improved.

The developmental processes studied occur in widely separated taxa, a fact which leaves one to imagine a simple determinism. In studying the adaptive and morphological evolutionary value, they should be put along side other traits: morphological, anatomical and biological factors (plant annual or perennial, etc., development of underground stems). The study of the ontogenesis of leaves based on Turlier (1985)’s work would be of interest in comparing both types of tillering ; the possibility of distinguishing each type may be connected with the width of the leaf base.

The results presented here are based on observations of buds positioned normally. With some species, particularly Monocotyledonae, extra-axillary branching (or more precisely that arising from an axillary situation as interpreted by Bugnon 1966) is only mentioned anecdotally, even though its anatomical aspect has been well studied by this author. The validity of the definitions put to test in Table 1 may be worth examining in plants possessing extra-axillary buds. Such studies of developmental morphology might be undertaken, at least in most typical situations, to improve our knowledge of the significance of distinguishing between different types of tillering.

Acknowledgements

Thanks to F. Bugnon (†) & M. F. Turlier (University of Dijon, France) for providing information, V. Malecot (Institut National d'Horticulture, Angers, France) for remarks, and the translator.

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