Christmas trees and other festive plants

David R. Hershey dh321 at PGSTUMAIL.PG.CC.MD.US
Wed Nov 29 00:13:01 EST 1995

On 26 Nov 1995, C.A.CORNFORD wrote:

> Hi
> I am preparing a public lecture on  plants associated with 
> Christmas. My aim is to use the occasion to feature some underlying biology 
> eg. use mistletoe to talk about parasitism, christmas flowering 
> plants for photoperiodism, juvenile and adult leaf forms etc. If 
> anyone has any interesting anecdotes, examples and 
> suggestions, local customs etc, please email me. 
> Many Thanks 
> Clive
> Clive Cornford
> Department of Applied Biology
> University of Central Lancashire
> Preston PR1 2HE
> Tel (01772) 893509
> Fax (01772) 892903
> Home Tel/fax (01323) 845536

Poisonous plants is a good topic because several Christmas plants are 
poisonous (holly fruit, mistletoe). Poinsettia is often reported to be 
poisonous but is not toxic as research has demonstrated.

Holly can be used as an example of a dioecious species and also as an 
example of parthenocarpy because gardeners often grow parthenocarpic 
cultivars to ensure fruit production. This eliminates the need for 
nonfruiting staminate plants or the grafting of a few staminate branches 
onto the pistillate trees. Another dioecious Xmas plant is the 
vine bittersweet (Celastrus scandens).

The Christmas poinsettia can be used for many biological topics. It is
very prone to molybdenum deficiency, yet molybdenum deficiency is often
difficult to induce in other species experimentally. The showy parts of
the "flower" are bracts.  In the US, poinsettia is the number one potted
flowering plant, even though it is only sold at Xmas. Aztecs first
cultivated the poinsettia, which was introduced to the United States in
1825 by Joel Robert Poinsett, first U.S. ambassador to Mexico. Chemical
growth retardants are often used on poinsettia, with Cycocel (chlormequat)
the most widely used. Poinsettia is propagated by cuttings treated with
the auxin, indole butyric acid. Poinsettias left in their sleeves too long
may exhibit epinasty (leaf drooping) due to ethylene production induced 
by sleeving. Poinsettias are very susceptible to white flies and root 
rots, the latter treated by soil drenches of fungicides. 

Much of the popularity of the poinsettia is due to selection of superior
cultivars which hold their bracts and leaves for months in home
environments.  The 1949 manual by Post noted that potted poinsettias often
dropped their lower leaves before bloom so asparagus fern or other plants
were planted in the same pot to hide the bare stems. Seedlings of silk oak
(Grevillea robusta) were also often used. 

David R. Hershey
Biology/Horticulture Dept.
Prince Georges Community College
Largo, MD 20772-2199

dh321 at

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