The _ovules_ are within the ovary and are arranged in different ways in different plants. The pollen grain (or better pollen-spore = microspore), after it has been transferred to the stigma, “germinates,” and the pollen tube grows down through the tissue of the stigma and style, or courses down the stylar canal until it reaches the ovule. Here it usually enters the ovule (macrosporangium) at the _micropyle_ (in some of the ament-bearing plants it enters at the _chalaza_), and the sperm cells are emptied into the embryo sac in the interior of the ovule. =810. Fertilization.=—One of the sperms unites with the egg in the embryo sac. This is _fertilization_, and from the fertilized egg the young embryo is formed still within the ovule. _Double fertilization_,—the other sperm cell sometimes unites with one or both of the “polar” nuclei which have united to form the “definitive” or “endosperm” nucleus. As a result of fertilization, the embryo plant is formed within the ovule, the coats of which enlarge by growth forming the seed coats, and altogether forming the seed. (See Chapters XXXIV, XXXV, XXXVI.) II. Kinds of Flowers. =811. Absence of certain flower parts.=—The _complete_ flower contains all the four series of parts. When any one of the series of parts is lacking, the flower is said to be _incomplete_. Where only one series of the floral envelopes is present the flowers are said to be _apetalous_ (the petals are absent), examples: elm, buckwheat, etc. Flowers which lack both floral envelopes are _naked_. When pistils are absent but stamens are present the flowers are _staminate_, whether floral envelopes are present or not; and so when stamens are absent and pistils present the flower is _pistillate_. If both stamens and pistils are absent the flower is said to be _sterile_ or _neutral_ (snowball, marginal or showy flowers in hydrangea). Flowers with both stamens and pistils, whether or not they have floral envelopes, are _perfect_ (or hermaphrodite), so if only one of these sets of _essential organs_ of the flower is present the flower is _imperfect_, or _diclinous_. Sometimes the imperfect, or diclinous, flowers are on the same plant, and the plant is said to be _monœcious_ (of one household). When staminate flowers are on certain individual plants, and the pistillate flowers of the same species are on other individuals, the plant is _diœcious_ (or of two households). When some of the flowers of a plant are diclinous and others are perfect, they are said to be _polygamous_.