Stamen initiation

Centripetal versus centrifugal stamen initiation in polystemonous androecia... 

The subclass Dilleniidae was introduced into angiosperm macrosystematics by Takhtajan (1964) largely influenced by the occiurence of centrifugal stamen initiation in polymerous androecia, which was supposed to be a fundamentally important pattern in macrosystematics. However, the subclass was later dismantled, first by structural cladistic studies (Hufford, 1992) and then also by molecular studies (Chase et al., 1993) (see also Endress et al., 2000). From the present perspective, the feature is not stable at very high systematic levels, however, often it is still at family level. 

Triuridaceae (Pandanales) Centrifugal carpels in fascicles central stamens initiated first in Lacandonia Rudall, 2008... 

Uhl, N. W. and Moore, H. E. (1977). Centrifugal stamen initiation in phy telephantoid pahns. American Journal of Botany, 64,1152-1161. 

Uhl, N. W. (1976). Developmental studies in Ptychosperma (Palmae). II The staminate and pistillate flowers. American Journal of Botany, 63,97-109. Uhl, N. W. and Moore Jr, H. E. (1977). Centrifugal stamen initiation in phytelephantoid palms. American Journal of Botany, 64,1152-1161. 

Fig 11.5 A. celastrifolia, stamen initiation. Sepals and petals removed in all. (A) Frontal view of androecial ring-wall with first stamens formed (asterisks) alternating with the petals. (B) Lateral view of a hud with first formed stamens (asterisks). (C) Androecial ring-wall (circle) and first formed stamens (arrowheads) alternating with the petals. (D) Side view of somewhat older hud. Stamen primordia are formed in a centripetal pattern. 

E) Older floral hud with massive androecial ring-wall (circle) distinctly separated from inner part of the flower. (F) Side view of androecial ring-wall. Arrows show the direction of stamen initiation. In the centre of the flower four carpel primordia are just formed (c). 

Fabales (Leguminosae) Pisum sativum, Swartzia spp. Pisum carpel initiated Sattler, 1972b after first (abaxial) Tucker, 2003 stamen primordia. Prenner, 2003, Centrifugal stamens in 2004a, b some Swartzia spp. Carpel before stamens in some Papilionoideae ... 

Malvales Bixaceae, Cistaceae, Cochlospermaceae, Malvaceae s.l. Many polyandrous genera Mostly intrazonal centrifugal stamens often in fascicles sometimes temporal overlap with carpel initiation Corner, 1946 Van Heel, 1966 Tucker, 1999 [Pavonia-Malvaceae)... 

Fig 9.5 Flower morphology of Heterocentron elegans (A)-(B), Clidemia octona (C)-(F) and of Dissotis rotundifolia (G)-(ff). (A) Mature flower bud with four petals (removed) and eight stamens. (B) Detail of flower showing a style surrounded by an extension of the ovary wall at the base. The anthers are provided with basal auriculate appendages. (C) Development of sepal lobes surrounding petal primordia. (D) Initiation of stamens and carpels. Note the... 

The discovery that in A. celastrifolia carpel initiation starts before the last stamens are formed, confirms Endress (1994, p. 103) observation that early carpel initiation is a characteristic of species in which stamens are formed on a ringlike androecial mound. However, early carpel formation is also frequently found in Papilionoideae with only ten stamens in two whorls (e.g. Daviesia cordata, Prenner, 2004d) and in Caesalpinioideae (c.f. Prenner and Klitgaard, 2008). 

It is demonstrated that the corona represents a complex structure, which is part corolla and part androecium, represented by two series of distinct staminodes. The argument that the flower is apetalous because of the strong resemblance and association of outer petaloid appendages to the stamen tube gains little support because of the initiation of petal lobes before the androecium and from the fact that petals and stamens are connected by a common stamen-petal tube, which is widespread in asterids. A floral developmental study of genera of Scytopetaloideae, Crateranthus and Astemnthos is expected to confirm the earlier initiation of the outer corolla before the centrifugal development of the androecium. 

My observations clearly demonstrate the sequential initiation of stamen and staminode primordia as separate pairs, not as fascicles. There is no evidence of a different growth rate between adjacent half-anthers and pairs of staminodes, suggesting that they belong together. 

El Hinnawy (1956) has reviewed the relation of macronutrient nutrition to flower initiation. He concluded that variations in macronutrient levels may greatly influence the time of appearance of floral buds, the rate of subsequent floral development, the number of flowers produced and the stamen—pistil ratios but there was no alteration in initiation of floral primordia in photoper-iodically sensitive plants. 

The pistil is the last of the floral structures to be differentiated during flower formation. In some dioecious species, such as Lychnis dioica, pistil initiation is linked to the presence of a sex chromosome (Westergaard 1958). In many species, however, sexual expression is more subject to modification by environmental factors acting on metabolic processes. This aspect has been studied frequently in monoecious members of the Cucurbitaceae. In these species, as in other monoecious types, light and temperature are particularly potent factors influencing the production of stamens and/or pistils. 

---