The Holly is a shrub with tree-potential though it is so humble that it will remain a shrub where other species have already formed a forest of trees. Unlike most other shrubs and bushes, Holly seems to exhibit an inner impulse for "uprightness": it forms a central stem with clearly defined side-branches whenever it is allowed to. The shape of the 10-15 m (30-45 ft.) high tree is often almost cylindrical.
Like other Iron Age Europeans, the Celts were a polytheistic people prior to their conversion to (Celtic) Christianity. The Celts divided the year into 13 lunar cycles (months or moons). These were linked to specific sacred trees which gave each moon its name. Today commences the Celtic tree month of Tinne.

Hollies are shrubs and trees of the genus Ilex. Many of them are highly decorative. Some have leaves with widely-spaced, spine-tipped leaves, while others have simple leaves.
Holly plants are male and female. Only the female has berries, but requires a male as a polleniser. Also bees are required as wind pollination is negligible.

"The Holly is male, and so symbolizes paternity and fatherhood. With the Ivy and the Mistletoe, the Holly has always been regarded as a potent life symbol, both for his year-long foliage and for his winter fruits. Concealed within the verses of the 'Song of Amergin', chanted by a chief Bard as the landed on the shores of Ireland, is the line 'I am a battle-waging spear' – the wood of the Holly was used in fashioning spear shafts. Magical Associations: Protection, prophecy, magic for animals, sex magic."

The holly and the ivy

"Traditionally at Christmas time a man was dressed up and covered in Holly branches and leaves, and a woman was likewise dressed in Ivy (the female counterpart of Holly). Together they would be paraded through the streets hand in hand leading the old year into the new. This is symbolic of the fertile interaction of the goddess and god during natures decline and the darkest time of the year, from which the new light of the sun-god springs forth encouraging fresh growth and renewed vegetation during the coming new year. Today the Holly King has been stylized by the figure of Santa Claus."








source :
http://www.flowersociety.org/Holly_plant_study.htm
http://www.wilsonsalmanac.com/book/jul8.html