PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 7. THE FLOWER AS A SPECIAL REPRODUCTIVE ORGAN OF ANGIOSPERMS

7.5. Flower Development in Ontogeny

Throughout their ontogeny, angiosperms undergo a series of successive changes that lead to The formation of various morphophysiological structures. Based on The Study of ontogeny in various representatives of angiosperm plants, F. M. Kuperman identified 12 stages of Organogenesis in monocots, 8 of which are associated with Flower Formation and functioning.

The GROWTH AND DEVELOPMENT of various structures in flowering plants typically occur due to The activity of the apical Meristems of the SHOOT apex. As a result of apical meristem activity, vegetative buds appear first, followed by reproductive (floral) buds. The specific factors that trigger the transition of the shoot apex from producing vegetative buds to forming floral ones—the transition from the vegetative to the reproductive sphere—remain not fully understood. While several hypotheses have been put forward, none of them is entirely convincing. What remains certain is that these critical developmental changes in plant ontogeny are driven by underlying biochemical and physiological factors.

Morphological changes observed during flower ontogeny show that the initiation of its elements first alters the shape of the shoot apex, transforming it from conical into a more or less flat or slightly concave receptacle. All floral elements originate on the receptacle in a specific order as meristematic tissue protuberances (Fig. 164). Typically, each protuberance initially forms separately, but zones of intercalary meristem may subsequently develop, leading to the fusion of individual floral elements. The earlier the zone of intercalary growth is established, the more fused the various elements will be. This is particularly evident in The Structure of the calyx and corolla.

Class="center">Fig. 164. Flower development in ontogeny: A—actinomorphic with a superior Ovary in violet wood sorrel (Oxalis violacea); B—zygomorphic with an inferior ovary in sweet mignonette (Reseda odorata): 1—shoot apex (future receptacle); 2—sepal protuberances of the calyx; 3—petal protuberances of the corolla; 4—stamen protuberances (initiation of the androecium); 5—protuberance forming the gynoecium

The sepal protuberances are the first to form on the receptacle. Initially, they grow uniformly in length, width, and thickness, but soon, due to more intensive Cell Division on the outer side, the protuberance changes shape and becomes convex-concave. Through this morphological shift, the developing sepals gradually approach one another and close up, forming a bud. The Development of petals proceeds in the exact same manner, initiating right after the sepals. An air gap remains in the bud between the sepals and petals, which enhances the thermal insulation Properties of the perianth. Due to these features of perianth formation, favorable conditions for the development and efficient functioning of the flower's reproductive elements—stamens and pistils—are established at very early developmental stages.

Stamens, much like sepals and petals, originate as meristematic tissue protuberances. During ontogeny, the anther differentiates first, while the filament develops through the activity of intercalary meristem.

A distinct pattern is observed during the Formation of the pistil. In some plants, the carpel that forms the pistil also initiates as a meristematic tissue protuberance, which gradually enlarges, changes shape, rolls up, and ultimately fuses along its margins. However, in plants with a syncarpous gynoecium, a congenital (from Lat. congenitus — innate) pistil most commonly develops. Such a pistil is initiated from the very beginning as an annular structure, and its subsequent development involves only the differentiation of its constituent parts.

During ontogeny, stamens and pistils develop not merely as morphological structures. Concurrently with the purely morphological processes that shape the stamens and pistils—and, accordingly, the androecium and gynoecium of the flower—embryological processes take place. In other words, within specific structures of the stamens and pistils, as will be discussed in detail later, strictly reproductive processes occur: spore formation, gametophyte development, formation of Germ Cells (Gametes), etc.

The shape of the future flower depends on the developmental characteristics of these meristematic protuberances. If the receptacle and the meristematic protuberances arising on it develop uniformly, the resulting flower will be actinomorphic. When the receptacle and meristematic protuberances grow unevenly, a zygomorphic flower is formed.