The seeds do not only carry within them the full-grown plant itself but they also symbolise the continuity of life: they will grow into wheat and the wheat will transform into a human body. The period preceding the Winter Solstice is a time to honour the pregnant Mother Earth: all living things enter her womb to grow and wait in darkness to be born again.
Capricorn is the tenth sign of the zodiac. Its period corresponds to the Winter Solstice, rich in symbolism of the interplay (and often struggle) between light and darkness. It is also strongly connected to all kinds of fertility rites. Its pagan connotations and its tie to the earth and the underworld are among the reasons why Christianity linked this sign to the Devil.