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The word pistil (from Latin pistillum meaning pestle) is also sometimes used to describe each discrete unit of the gynoecium. A pistil can consist of either a single carpel (in a monocarpous or apocarpous gynoecium), in which case it is called a simple pistil, or of several fused carpels (in a syncarpous gynoecium), in which case it is called a compound pistil.The style and stigma of the flower are involved in most types of self incompatibility reactions. Self incompatibility, if present, prevents fertilization by pollen from the same plant or from genetically similar plants, and ensures outcrossing.
Stigmas can vary from long and slender to globe shaped to feathery. The stigma is the receptive tip of the carpel, which receives pollen at pollination and on which the pollen grain germinates. The stigma is adapted to catch and trap pollen, either by combining pollen of visiting insects or by various hairs, flaps, or sculpturings. Stigmas must distinguish and reject the pollen of other species, and in some cases are responsible for self incompatibility.
The style of a pistil is the tube-like portion between the stigma and the ovary. It can be either long or short. In some cases the style is responsible for self incompatibility, causing pollen tubes to fail.The ovule (from Latin ovulum meaning small egg) is a complex structure, born inside ovaries of carpels in angiosperms. The ovule initially consists of a stalked, integumented megasporangium. Typically one cell in the megasporangium undergoes meiosis resulting in one to four megaspores. These develop into reduced megagametophytes (often called embryo sacs) within the ovule. Before fertilization, the ovule consists of one or two layers of integuments surrounding the remains of the megasporangium, called the nucellus and an embryo sac, with a small number of cells and nuclei, including one egg cell and two polar nuclei (which will form, together with a sperm cell, the primary endosperm nucleus). The gap in the integuments through which the pollen tube enters to deliver sperm to the egg is called the micropyle. The stalk attaching the ovule to the placenta is called the funiculus. Ovules are typically positioned so that the micropyle is facing the point of funiculus attachment, but other positions are found in a variety of plant groups.
February 25th, 2012
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