Morphological Development of Flowers

Organogenesis of Flowers

The diversity, complexity and dynamics of floral development


Rolf Sattler

My Contributions and General Issues


Floral development, especially the development of floral organs (organogenesis), has been studied for centuries. In 1857 (two years before Charles Darwin’s
On the Origin of Species) the French botanist Jean-Baptiste Payer published the monumental Traité d’organogénie comparée de la fleur. As far as I can see, this work remains the most comprehensive on floral development (organogenesis) up to the present time. It has, however, some shortcomings for which Payer cannot be blamed. Since microscopic photography was not yet sufficiently developed at that time, he could provide only drawings. And since the resolution and magnification of microscopes was rather limited, his observations were not always free of errors. However, overall they appear remarkably and admirably accurate.

In the 20
th century in 1973 I published the Organogenesis of Flowers. A Photographic Text-Atlas. (Since this book has much in common with Payer’s, some of my French colleagues called me the Payer of the 20th century or the modern Payer) My book is not as comprehensive as Payer’s: it describes the floral development of only 50 different species of flowering plants, whereas Payer’s Traité includes over three hundred. But as the title of my book indicates, it documents the stages of floral development photographically by a special epi-illumination technique that I developed in my laboratory (Sattler 1968). This technique displays floral development in three dimensions (photograph taken by Dr. Usher Posluszny). Posluszny, Scott, and Sattler (1980) have further improved this technique, and Dadpour et al. (2008) developed a digital version. Dr. Somayeh Naghiloo used this digital version in her research on floral development and because of the astounding beauty of her photos won awards in international photo competitions.

Although I am the author of
Organogenesis of Flowers, I have to emphasize that many students and technicians have greatly contributed to it. Dr. Alastair D. Macdonald contributed 6 chapters and Ms. Vera Block 8. Other students and technicians also made important contributions. I am most grateful to all contributors (see Acknowledgements in the book).

As far as I know, so far nobody has published a photographic text-atlas on floral development that is more comprehensive than mine. But many papers have been published on the floral development of a great variety of species and many of these papers document the stages of floral development by means of scanning electron microscopy and/or microscopic sections. For an incomplete bibliography see, for example, Greyson (1994) and Leins & Erbar (2010).

The flower has been defined in different ways and thus different floral concepts have been proposed, which has led to much controversy (see, for example, Macdonald and Sattler 1973). Being unaware of or ignoring this controversy and complexity, in most textbooks and research papers a flower is simply defined as consisting of an axis with sterile and fertile appendages that are considered homologues of leaves. The sterile appendages form the perianth that often consists of sepals and petals or tepals (and sometimes may be absent). The fertile appendages are the stamens (microsporophylls) and carpels (megasporophylls) that carry and enclose the ovules (but see Brückner (2000) for many different definitions of the carpel concept).

This common definition of the flower appears rather limited in view of the observed diversity and complexity of floral development. In the androecium, stamens cannot be generally equated with microsporophylls (fertile phyllomic appendages) because at their inception on the floral apex they present a continuum from flat (phyllomic) to radial (caulomic) structures. Only the phyllomic stamens can be called microsporophylls, the others along the continuum resemble microsporophylls less and less as they become more and more caulomic (stem-like). The continuum also extends towards branchlets (fertile shoots) in androecia in which common primordia give rise to stamen fascicles (Rutishauser and Sattler 1985, Sattler 1988, Sattler and Jeune 1992, Leins and Erbar 2010). Therefore, it seems debatable whether all flowers have only one axis. At least some flowers may be more or less polyaxial.

In the gynoecium, ovules are not always born on appendages (carpels), but in some species may arise on the floral axis. This means that besides the typically carpellate flowers acarpellate flowers can be distinguished (Sattler 1974, Brückner 1991, 2000, p. 180). If, however, we change the traditional definition of the carpel as an appendage that encloses and bears ovule(s) and redefine it as an appendage that encloses ovule(s) and may or may not bear them, then most (but not all) of the acarpellate flowers can be considered carpellate. We proposed this redefinition long ago (
Sattler and Perlin 1982) and alluded to it again later (Sattler and Lacroix 1988). Greyson (1994) and Leins & Erbar (2010) also used this redefinition. It simplifies matters, but it must be clear that according to this redefinition a carpel can no longer be generally equated with a megasporophyll because it may or may not be a megasporophyll depending on whether it bears or does not bear the megaspore producing ovule(s). Furthermore, it must be clear that even according to the redefinition of the carpel, there are still gynoecia without carpels (acarpellate gynoecia) (Sattler1974). Examples of such acapellate gynoecia can be found in Stylidium and Balanophora (Sattler 1973, 1974). I have discussed evolutionary developmental processes through which these and other patterns may have arisen (Sattler 1974, 1992).

The protective enclosure of ovules constitutes a characteristic feature of flowering plants, which for this reason have been called angiosperms, plants with enclosed seeds, in contrast to gymnosperms with exposed seeds. It needs to be stressed that the enclosure of the ovules and seeds has been achieved through several different ways. Only one of these ways involved the formation of the classical carpel (megasporophyll). Another way was the formation of gynoecial appendages that enclosed ovules but did not bear them. And yet other ways have been reported in the literature (see, for example, Sattler 1974). Brückner (2000, pp. 176-180) noted ten different interpretations of the gynoecium involving different carpel concepts. See also Lorch (1963) for a historical review of carpel concepts and problems engendered by these concepts.

Considering the enormous diversity and complexity that have been documented by many authors including us, the usefulness of the common simplistic definition of the flower as an axis that bears sterile and fertile phyllomes (stamens and/or carpels) appears rather limited. Although many appendages found in flowers appear phyllomic (leaf homologues), others may be caulomic (stem homologues) or more or less intermediate between phyllomes, caulomes and other categories such as branches. Therefore, understanding the enormous structural diversity in terms of a
continuum between these categories appears closer to reality than a purely categorical approach.

Claßen-Bockhoff (2016) also concluded that "the shoot concept of the flower...should be questioned." And she proposed that, "flowers are sporangia bearing units rather than short shoots with floral organs" because "from the functional point of view, stamens and carpels are sporangiophores and as such 'de novo' structures not necessarily homologous with vegetative leaves." As pointed out above, the androecial sporangiophores are more or less highly branched. For example, those that form stamen fascicles are more highly branched than those that form individual stamens. The gynoecial sporangiophores are more or less or not at all integrated with the gynoecial appendages that enclose them (that in classical morphology have been called carpels (megasporphylls), but since they do not always bear the sporangia, they are not always megasporophylls as pointed out above).

According to the theory of Anaphytes (Schultz 1843) flowers are branching systems that result from the processes of branching and articulation. Thus, there is no need to categorize them in terms of phyllomes, caulomes, and shoots. Branching is understood in the wide sense as the formation of a new primordium that then develops into a new article or segment. This theory appears compatible with
process morphology, especially if the articles between two branching points are understood as process combinations (see below). For this theory it is not necessary that the structures of the flower are reduced to the categories of classical morphology such as caulome (stem) and phyllome (leaf), nor does it require reference to 'de novo' structures because according to this theory each different structure is more or less 'de novo;' therefore no fundamental distinction is made between structures that are 'de novo' and those that are more or less the same.

One reason, among others, why the common simplistic definition of the flower as a modified short shoot persists to a great extent: patterns that contradict this simplistic definition are forced to conform to it by means of the concept of “congenital fusion.” But this kind of “fusion” refers to a variety of processes, some of which misrepresent developmental and evolutionary processes, referring to fusion where none is observable (Sattler 1974, 1978). Instead one or several of the following developmental and evolutionary processes may have been operative:
1. Change of the relative position of primordial inception (spatial shifting), which leads to heterotopy (that may be related to ectopic gene expression);
2. Change of timing (temporal shifting), which implies heterochrony;
3. Zonal growth (intercalary growth) and other modifications, which lead to heteromorphy (Zimmermann 1959, Sattler 1992).
Using the concept of congenital fusion may obscure these processes, which means it may obscure the hologenetic view of evolution (Zimmermann 1959) according to which evolution needs to be understood as a change in the development of organisms as it is emphasized in evolutionary developmental biology (
evo-devo). For these reasons, I have proposed to replace the concept of congenital fusion by concepts that refer to processes that are at least in principle observable (Sattler 1974, 1978). Some authors such as Greyson (1994) and Leins & Erbar (2010) have adopted my proposal (see also Leins 1972 and Leins, Merxmüller and Sattler 1972).

Like almost all research on floral development, most of our research implied a structure-process dichotomy, which means that structures and processes are distinguished: structures exhibit processes. However, using data from floral development, I introduced a process morphology that transcends this dichotomy because according to process morphology structures do not have processes, they
are processes (Sattler 1988). Thus, floral morphology can be understood in terms of process morphology that demonstrates a dynamic continuum.

Nowadays, in our genocentric age, floral development is investigated mainly in terms of molecular genetics. Although these investigations can shed some light on floral development, they are limited for at least two reasons: 1. So far only relatively few species have been analyzed in terms of molecular genetics, and 2. Flowers are much more than just genes. Therefore, descriptive and comparative studies of the diversity, complexity and dynamics of floral development remain important for a more complete understanding of flowers.

We also have to keep in mind that whatever we describe through language and/or mathematics is somewhat removed (abstracted) from reality. Words are not the things they refer to; language and mathematics, like a map, are not the territory they refer to as has been clearly pointed out by
Korzybski through his structural differential. Ultimate reality remains fundamentally unspeakable. Hence the importance of silence (see also From Plant Morphology to Infinite Issues (including Ken Wilber and Korzybski).

Buddha’s “
flower sermon” also underlines the importance of silence. It has been said that one day, as usual, a crowd had gathered to listen to the Buddha. But he did not speak, he held a flower in his hand. The crowd became rather restless as he kept looking at the flower in silence. Finally, Mahakashyap, one of his disciples, started laughing. Then Buddha gave him the flower and said to the crowd: “Whatever can be said through words, I have said to you, and that which cannot be said through words, I give to Mahakashyap. The key cannot be communicated verbally...What is the key? Silence and laughter is the key - silence within, laughter without. And when laughter comes out of silence, it is not of this world, it is divine” (Osho: Meditation. The first and the last freedom, pp. 190-191).



References

Brückner, C. 1991. Zur Interpretation des Karpells - eine Übersicht. Gleditschia 19: 3-14.

Brückner, C. 2000. Clarification of carpel number in Papaverales, Capparales, and Berberidaceae. Botanical Review 66 (2): 155-307.

Claßen-Bockhoff, R. 2016. The shoot concept of the flower: Still up to date? Flora 221: 46-53.

Dadpour M. R., Grigorian W., Nazemieh A., Valizadeh M. 2008. Application
of epi-illumination light microscopy for study of
floral ontogeny in fruit
trees. International Journal of Botany 4, 49
55.
doi:
10.3923/ijb.2008.49.55

Greyson, R.I. 1994.
The Development of Flowers. New York/Oxford: Oxford University Press.

Leins, P. 1972. Das Karpell im ober- und unterständigen Gynoeceum. Berichte der deutschen botanischen Gesellschaft 85: 291-294.

Leins, P., Merxmüller, H. and Sattler, R. 1972. Zur Terminologie interkalarer Becherbildungen in Blüten. Berichte der deutschen botanischen Gesellschaft 85: 294.

Leins, P. and Erbar, C. 2010.
Flower and Fruit. Stuttgart: Schweizerbart Science Publishers.

Lorch, J. 1963. The carpel - a case-history of an idea and a term. Centaurus 8: 269-291.

Macdonald, A. D. and Sattler, R. 1973. Floral development of
Myrica gale and the controversy over floral concepts. Canadian Journal of Botany 51:1965-1976.

Payer, J.-B. 1857. Traité d’organogénie comparée de la fleur. Paris: Masson.

Posluszny, U., Scott, M.G. and Sattler, R. 1980. Revisions in the technique of epi-illumination lightmicroscopy for the study of floral and vegetative apices. Canadian Journal of Botany 58: 2491-2495.

Osho. 2004. Meditation. The first and the last freedom. New York: St. Martin’s Griffin.

Rutishauser, R. and Sattler, R. 1985. Complementarity and heuristic value of contrasting models in structural botany. 1. General considerations. Botanische Jahrbücher für Systematik 107: 415-455.

Sattler. R. 1968. A technique for the study of floral development. Canadian Journal of Botany 46: 720-722.

Sattler, R. 1973.
Organogensis of Flowers. A Photographic Text-Atlas. Toronto: University of Toronto Press.

Sattler, R. 1974. A new approach to gynoecial morphology. Phytomorphology 24: 22-34.

Sattler, R. 1978. “Fusion” and “continuity” in floral morphology. Notes of the Royal Botanic Garden, Edinburgh 36: 397-405.

Sattler, R. 1988. A dynamic multidimensional approach to floral development. In: Leins, P., Tucker, S. C. and Endress, P. K. (eds.)
Aspects of Floral Development. Berlin: J. Cramer/Borntraeger, pp. 1-6.

Sattler, R. 1992. Process morphology: structural dynamics in development and evolution. Canadian Journal of Botany 70: 708-714.

Sattler, R. and Perlin, L. 1982. Floral development of
Bougainvillea spectabilis Willd., Boerhaavia diffusa L. and Mirabilis jalapa L. (Nyctaginaceae). Botanical Journal of the Linnean Society 84: 161-182.

Sattler, R. and Lacroix, C. 1988. Development and evolution of basal cauline placentation in
Basella rubra. American Journal of Botany 75: 918-927.

Sattler, R. and Jeune, B. 1992. Multivariate analysis confirms the continuum view of pant form. Annals of Botany 69: 249-262.

Schultz, C. H. 1843. Die Anaphytose oder Verjüngung der Pflanzen. Ein Schlüssel zur Erklärung des Wachsens, Blühens und Fruchttragens, mit praktischen Rücksichten auf die Kultur der Pflanzen. Berlin.

Zimmermann, W. 1959. Die Phylogenie der Pflanzen. 2nd edition. Jena: Gustav Fischer Verlag.

See my
Publications for further references on investigations of floral development carried out in my laboratory such as those by Sattler (1962, 1967, 1972), Cheung and Sattler (1967) Macdonald and Sattler (1973), Posluszny and Sattler (1973, 1974, 1976), Singh and Sattler (1972-1977), Sattler and Singh (1973-1978), Pauzé and Sattler (1978, 1979), Lacroix and Sattler (1988), and Lehmann and Sattler (1992-1997)




Latest update of this webpage on September 8, 2017

Home