This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Except in minor details this is accomplished much in the same way in all vascular plants, which include Ferns, Selaginellas, and their allies. The pollen grains of a flowering plant are equivalent to the microspores of a Selaginella; and the anther in which they are produced to the microsporangium of Selaginella. The germinal vesicle or egg cell is equivalent to the megaspore, and the embryo sac in which it is produced to the megasporangium of a Selaginella. The pollen is the male element and the egg cell the female. A pollen grain has two coats or skins, and when it reaches the stigma the inner coat soon after protrudes and grows down the loose, conducting tissue in the interior of the stigma and style, in the form of a blind or closed tube (fig. 54), then passes into the ovary, and enters the micropyle or opening of the ovule until it comes in contact with the egg cell (fig. 56). The growing or elongating pollen tube contains two nuclei, and the first and larger one fuses with the nucleus of the egg cell and fertilization is complete. In other words, the male and female elements have united to form one cell, which forthwith develops into an embryo or new individual. If the pollen has been brought from another plant of the same kind, the embryo will inherit the characters of both parents, and, though it may not seem to differ much from either when it grows into a plant, it is usually more vigorous than an embryo which is the result of self - fertilization. This is sexual reproduction, and is vastly different from the increase of numbers secured by cuttings, layers, and offsets, which are merely pieces of the individual from which they were taken.
Fig. 54 - Flower of Cistus: Sepals and Petals removed. The Stamens are hypogynous, and some of them have their Anthers in contact with the Stigma. The Pollen Tubes are shown passing down the Style and entering the Ovules.