The stamens and the pistil are the _essential_ organs of a flower, because they, and they only, are needed in the formation of seeds. The pollen from the stamen, acting on the pistil, causes the _ovules_ which are in the pistil to grow into _seeds_. The calyx and corolla are called _enveloping organs_, since they surround and protect the essential parts. The pieces of which the calyx is composed are called _sepals_. The Apple-blossom has five sepals. The pieces that compose the corolla are called _petals_. KINDS OF FLOWERS.--When the petals are entirely separate from each other, as in the Apple-blossom, the flower is said to be _polypetalous_; when they grow together more or less, as in the Catalpa (Fig. 10), _monopetalous_; and when the corolla is wanting, as in the flowers of the Oak, _apetalous_. [Illustration: Fig. 10.] When all sides of a flower are alike, as in the Apple-blossom, the flower is _regular_; when one side of the corolla differs from the other in color, form, or size, as in the Common Locust, or Catalpa, the flower is _irregular_. In trees the stamens and pistils are often found in separate flowers; in that case the blossoms containing stamens are called _staminate_, and those containing pistils _pistillate_; those that contain both are called _perfect_. Staminate and pistillate flowers are usually found on the same tree, as in the Oaks, Birches, Chestnut, etc.; in that case the plant is said to be _monoecious_, and all trees of this kind produce fruit. Sometimes, however, the staminate and pistillate flowers are on separate trees, as in the Willows, which are _dioecious_; and then only a portion of the trees--those with pistillate flowers--produce fruit. ARRANGEMENT OF FLOWERS.--Flowers, either solitary or clustered, grow in one of two ways; either at the end of the branches, being then called _terminal_, or in the axils of the leaves, then called _axillary_. The stem of a solitary flower or the main stem of a cluster is called a _peduncle_; the stems of the separate blossoms of a cluster are called _pedicels_. When either the flowers or the clusters are without stems, they are said to be _sessile_. _Clusters with Pedicellate Flowers._ _Raceme_, [Illustration] flowers on pedicels of about equal length, scattered along the entire stem. Locust-tree. _Corymb_, [Illustration] like a raceme except that the lower flowers have longer stems, making the cluster somewhat flat-topped; the outer flowers bloom first. Hawthorn. _Cyme_, [Illustration] in appearance much like a corymb, but it differs in the fact that the central flower blooms first. Alternate-leaved Cornel. _Umbel_, [Illustration] stems of the separate flowers about equal in length, and starting from the same point. Garden-cherry. _Panicle_, [Illustration] a compound raceme. Catalpa. _Thyrsus_, a compact panicle. Horse-chestnut. _Clusters with Sessile or Nearly Sessile Flowers._