All About Holly...and Ivy...

Holly and ivy, "Winter in Elmstead Woods, U.K."  Wikepedia. 

From Barbara G. Walker's The Woman's Encyclopedia of Myths and Secrets:

Holly

To the druids, holly was the plant of death and regeneration, sacred to Mother Holle, or Hel, the underworld Goddess.(1)  Germanic witches who worshipped her favored holly wood for magic wands.  Red holly berries showed the female blood-of-life color, corresponding to white mistletoe berries associated with male elements of semen and death.  In the divine marriage celebrated at Yule, they were displayed together.
The "holy" holly was linguistically linked with Hel's younic "hole" (Germanic Hohle, a cave or grave). It was the most sacred of trees, according to a carol sung by medieval pagans at Yuletide, saying holly "bears the crown."(2)
In the Dionysian cult, female holly was paired with the god's male symbol, ivy.(3)  Green boughs of both were used to adorn doorways at the solstitial festival.  Tertullian condemned the custom, saying any Christian who has "renounced temples" should not make a temple of his own house door.(4)  Nevertheless, house-decorating with holly, ivy, or mistletoe at the solstitial festival went serenely on.  The Council of Bracara ruled that no Christian should bring holly into his house for Christmas, because it was a custom of "heathen people."(5)  Heathen or not, it was inextricably linked with Yuletide celebrations and could not be eradicated.

Even the sexual symbolism of the holly was remembered, in a way, up to the 17th century.  Christmas games included a mock battle of the sexes, in which the master and mistress of the house engaged: "Great is the contention of the holly and ivy, whether master or dame wears the breeches."(6)  The kiss under the mistletoe originally represented sexual union, a peaceful resolution of the battle.

Notes:

(1)  Goodrich, 54.
(2)  Graves, W.G., 186.
(3)  de Lys, 63.
(4)  Miles, 269.
(5)  Hazlitt, 118, 127.
(6)  Hazlitt, 120.

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Lyrics to "The Holly and the Ivy"

The holly and the ivy, When they are both full grown, Of all trees that are in the wood, The holly bears the crown. Refrain: O, the rising of the sun, And the running of the deer The playing of the merry harp, Sweet singing in the choir. The holly bears a blossom, As white as lily flow'r, And Mary bore sweet Jesus Christ, To be our dear Saviour. The holly bears a berry, As red as any blood, And Mary bore sweet Jesus Christ, To do poor sinners good. The holly bears a prickle, As sharp as any thorn, And Mary bore sweet Jesus Christ, On Christmas Day in the morn. The holly bears a bark, As bitter as the gall, And Mary bore sweet Jesus Christ, For to redeem us all. Refrain The holly and the ivy, When they are both full grown, Of all trees that are in the wood, The holly bears the crown.

Meaning Behind "The Holly and the Ivy"
A Battle of the Sexes Played Out Through Plant Symbology
from About.com
By David Beaulieu