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.
When the ovule is fertilized by the union of the nucleus of the pollen tube with the egg cell it becomes a seed. At this stage it consists of three parts: (1) The testa, or skin, made of two layers or coats, and belonging to the mother plant; (2) the endosperm, a mass of tissue developed to nourish the embryo; and (3) the embryo or young plant. The endosperm and embryo are new developments, resulting from the act of fertilization. As the seed progresses to maturity it undergoes great changes, and, in many cases, remarkable developments (figs 59, 60). The testa may remain thin and membranous, especially in those cases where it is covered by the walls of the fruit at maturity and after it has fallen away from the mother plant, as in the Buttercup, Clematis, and Chrysanthemum, the fruits of which must not be mistaken for seeds. The testa also remains thin in the Peach and Cherry, where it is protected by the bony endocarp or stone. It becomes leathery, spongy, or fleshy in different species of Iris, and crustaceous in the Honeysuckle; while it may be much wrinkled, crustaceous, tubercled, or warted in species of Delphinium. Tubercled seeds may be found in Lychnis, Silene, and Stellaria; netted or pitted ones in Poppy, Passionflower, and their allies. In the Willow Herb (Epilobium) the seeds develop a long pencil of hairs at the top, and in the Willow and Poplar the parachute of hairs arises at the base; and in all cases the hairs are intended to assist the dissemination of the seeds by the wind. This, also, is the intention where the testa develops round the edges or at both ends into a membranous expansion or wing, as in the Bignonia and Moon-seed families, in the Stock, Arabis, and many others.
Fig. 58. - Section through the Fruit (Drupe) of a Plum, showing the Epicarp (ep) or Skin, the Sarcocarp (sar) or Flesh, the Endocarp (en) or Stone. In the centre is the solitary Seed or Kernel.
Fig. 59. - Seeds, showing the Outer Skin or Testa with rugged prominences or projections. The sections show the Seeds cut lengthwise, and show the Embryo with its two Cotyledons, and the Radicle surrounded by the Perisperm.
1, Rue (Ruta graveolens). 2, Snapdragon below.
As the seed matures, the endosperm grows and becomes fleshy, horny, or mealy in different families, and fills up the interior of the seed to a greater or less extent, or may disappear altogether, as in the Crucifers and Rose families, where the embryo uses it all up before maturity. The embryo may remain small or grow to fill the seed, and intermediate stages are very numerous. It may store all the food materials in itself and the cotyledons may remain thin or become fleshy. At maturity the seed assumes various colours, such as red, brown, white, or black, and these colours are a sign that growth has ceased. The testa dries up, but the endosperm and embryo remain alive and retain their protoplasm and other contents, though the embryo alone is capable of resuming growth in most cases. The Castor-oil seed is an exception, for the endosperm also grows during germination.