=Essential Organs=.—The essential organs are of two series. The outer series is composed of the =stamens=. The inner series is composed of the =pistils=. _Stamens_ bear the =pollen=, which is made up of grains or spores, each spore usually being a single plant cell. The stamen is of two parts, as is readily seen in Figs. 173, 174,—the enlarged terminal part or =anther=, and the stalk or =filament=. The filament is often so short as to seem to be absent, and the anther is then said to be _sessile_. The anther bears the pollen spores. It is made up of two or four parts (known as sporangia or spore-cases), which burst and discharge the pollen. _When the pollen is shed, the stamen dies._ [Illustration: FIG. 175.—THE STRUCTURE OF A PLUM BLOSSOM. _se_, sepals; _p_, petals; _sta_, stamens; _o_, ovary; _s_, style; _st_, stigma. The pistil consists of the ovary, the style and the stigma. It contains the seed part. The stamens are tipped with anthers, in which the pollen is borne. The ovary, _o_, ripens into the fruit.] The _pistil has three parts_: the lowest, or seed-bearing part, which is the =ovary=; the =stigma= at the upper extremity, which is a flattened or expanded surface, and usually roughened or sticky; the stalk-like part or =style=, connecting the ovary and the stigma. Sometimes the style is apparently wanting, and the stigma is said to be sessile on the ovary. These parts are shown in the fuchsia (Fig. 174). The ovary or seed vessel is at _a_. A long style, bearing a large stigma, projects from the flower. See also Figs. 175 and 176. Stamens and pistils probably are homologous with leaves. A pistil is sometimes conceived to represent anciently a leaf as if rolled into a tube; and an anther, a leaf of which the edges may have been turned in on the midrib. [Illustration: FIG. 176.—SIMPLE PISTILS OF BUTTERCUP, one in longitudinal section.] [Illustration: FIG. 177.—PISTIL OF GARDEN PEA, the stamens being pulled down in order to disclose it; also a section showing the single compartment (compare Fig. 188).] The pistil may be of _one part or compartment, or of many parts_. The different units or parts of which it is composed are =carpels=. Each carpel is homologous with a leaf. Each carpel bears one or more seeds. A pistil of one carpel is =simple=; of two or more carpels, =compound=. Usually the structure of the pistil may be determined by cutting horizontally across the lower or seed-bearing part, as Figs. 177, 178 explain. A flower may contain a simple pistil (one carpel), as the pea (Fig. 177); _several simple pistils_ (several separate carpels), as the buttercup (Fig. 176); or a _compound pistil_ with carpels united, as the Saint John’s wort (Fig. 178) and apple. How many carpels in an apple? A peach? An okra pod? A bean pod? The seed cavity in each carpel is called a =locule= (Latin _locus_, a place). In these locules _the seeds are borne_.