| Author||W.A. van Heel|
|Title||Morphology of the androecium in Malvales|
|Journal||Blumea - Biodiversity, Evolution and Biogeography of Plants|
|Abstract||In order to test the applicability of the telome and the classic theories to the nature of stamens a study was made of flowers in Malvaceae, Bombacaceae, Sterculiaceae, Tiliaceae, and Elaeocarpaceae with respect to their ultimate form, their development, and the vascular course of their androecia. The customary dissecting, clearing, and microtomy techniques were used. A chapter on the interpretation of the vascular bundle course in floral morphology has been added.|
In each flower the stamens are considered to be arranged in five groups, which may be fused more or less laterally. The groups have a three-trace vascular system the laterals of which may be commissural. Only a few Tiliaceae have ten groups.
Whereas in most Tiliaceae and Elaeocarpaceae the groups are antesepalous, in Malvaceae, Bombacaceae, and Sterculiaceae they are either antesepalous, antepetalous, or intermediate, depending on the variation of a spiral growth in the floral apex. If there is no such growth the groups are antepetalous. Spiral growth is revealed by an oblique course of the vascular bundle traces and the asymmetrical form of the petals and stamen groups.
The stamen groups may have many stamens (Tiliaceae) or bear fewer stamens in a fan-like arrangement, in which case they are called either staminal lobes (Malvaceae and Bombacaceae) or, if they are less welldeveloped, phalanges (Sterculiaceae and many Elaeocarpaceae). Since they have their own group primordia, which show a certain phyllotaxis, the stamen groups are considered to be individual in nature. Moreover, in Bombax and allied genera and in some Malvaceae the apical regions of the staminal lobes originate free and are phyllomic in appearance.
Three parts can be distinguished in each stamen group: one median upper and two lateral outer parts, corresponding with the three-trace vascular system. The upper part is often formed by a few stamens in Tiliaceae and by a single stamen or a staminodial part in Bombacaceae and Sterculiaceae; in Malvaceae it is reduced. Judging by the sub-group primordia, here called staminal buttresses, the lateral parts are considered to have a certain individuality; in some Malvaceae and very markedly so in Pachira spp. the staminal buttresses arise on the staminal lobes to form the lateral parts of the stamen groups. The parts have many stamens, arranged spatially (in Tiliaceae and Pachira spp.), or fewer stamens occurring as simple rows (mainly in Malvaceae and Sterculiaceae). Between the two an intergradated series of forms can be found, e.g., in Bombax and allied genera and in some Malvaceae. In some cases the lateral parts are formed by a single stamen or even a single theca.
The position of the carpels depends upon the position of the stamen groups. The carpels alternate with those regions of the stamen groups the development of which is the most advanced when the carpel primordia arise.
The parts of the groups as well as the groups as a whole may formerly have been freer but they have been reduced by congenital fusion and partial incorporation in the main floral axis. Evidence for this interpretation is provided by some Tiliaceae with ten freer stamen groups that may be visible either when mature (Mollia speciosa) or only in an early phase of development (Colona spp.). In Mollia speciosa the antepetalous groups are collaterally double, so that they may be regarded as the lateral parts of triple arrangements. In the Colona spp. the antepetalous groups are obscured during ontogeny by secondary receptacular growth. Moreover, if at the same time reduction occurs in the median line of these groups, their lateral parts are assimilated by the adjoining antesepalous groups. What can happen during ontogeny (postgenitally) may equally well happen (congenitally) before the primordia become visible externally.
The stamens may represent slightly flattened syntelomic structures. Depending on the time the division process begins, in Malvaceae and Bombacaceae a monothecous stamen primordium can be more or less deeply divided, forming two equal halves with facing xylem. In Malvaceae this dividing process may operate up to three times in succession; the products of the division are all arranged in one plane. Still further divisions may occur in Adansonia digitata and Durio spp.; in that case the results are arranged spatially, though in Durio not in an adaxial median direction. The mesomes occur either free, especially in Durio, or fused. This successive pairing of mesomes cannot be explained by connation of stamens. Under the influence of reduction superficial division gives rise to a bithecous stamen. Partitioned pollen sacs can be considered as due to imperfect division. The nature of several kinds of sterile extensions has been discussed. In Sterculiaceae and Tiliaceae the stamens of many species are resupinate.
In some species the lateral margins of the staminal lobes form only a few sessile thecae (Matisieae). In still others (Fremontia californica, Chorisia and Ceiba spp.) each margin immediately forms a single continuous theca each, the primordial staminal lobes being distinctly foliar in appearance. In this way each entire staminal lobe forms a single flattened bithecous stamen with a sterile apical part and a three-trace vascular system. It is precisely these characteristics that the classic theory regards as primitive in stamens. Moreover, it follows that in Mahales bithecous stamens can be structures of two different ranks.
Consequently in the androecia of this group of taxa a series of transitions exists between more branched and more foliar formations. In this series reduction operates by the loss of parts, by congenital fusion, by incorporation and by flattening. The androecial parts are neither fully stachyosporous nor fully phyllosporous; they are transitory between the two.
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