Please don't eat the poinsettias. Or the holly. And especially not the mistletoe.

The most popular plants families use to deck their halls at Christmastime are poisonous. And doctors say the holiday season is particularly dangerous for plant-loving children.

Plant eating is the most common cause of calls of poison control centers in Washington and elsewhere in the country.

Dr. Lewis Goldfrank, director of emergency services at New York's Bellevue Hospital and an expert on poison plants, said mistletoe is safe if it stays on the ceiling but can be lethal if its berries drop within a toddler's reach.

The leaves are harmless, but the berries can cause severe stomach pain, vomiting, diarrhea and -- in large quantities -- seizures, high blood pressure and fatal cardiac arrest.

Some local stores sell real mistletoe plants with plastic berries substituted for their toxic look-alikes.

Eating holly also can cause stomach pain, vomiting and diarrhea, Goldfrank said, but holly is less dangerous than mistletoe. Nonetheless, children who eat the plant's berries occasionally develop such severe symptoms that they become dehydrated and require hospitalization.

Poinsettias are toxic, too. But Goldfrank said they probably are less dangerous than past reports have suggested. The leaves and petals can cause vomiting, and the sap may irritate the skin and eyes, but Dr. Mark Fow said most poison control workers no longer consider poinsettias deadly.

Fow, a toxicologist at the National Clearinghouse for Poison Control Centers, said a review of 2,000 reports of children eating poinsettia plants between 1971 and 1978 showed only 150 of them developed any symptoms. The most serious problem reported was stomach irritation, Fow said.

The toxicologist said the widespread belief that poinsettias are deadly apparently dates from an unproven report in 1919 that a 2-year-old had died of poinsettia poisoning.

Commercially sold wreaths usually contain boughs of white pine, Pacific pine, balsam and juniper, according to a florist at Johnson's Flower Center on Wisconsin Avenue. The Poison Control Center at Children's Hospital lists white pine and balsam as nontoxic and says white pine is even a good source of vitamin C. Pacific pine is not listed. Juniper is not considered dangerous unless it is eaten continuously.

But yew, an evergreen shrub related to juniper, could make a wreath deadly. Animals that eat the berries soon stagger and have convulsions. "Half a dozen of them can kill a horse," Goldfrank said.

Fow said Jerusalem cherries, another ornamental Christmas plant, also cause serious poisoning.

Children under 3 years old account for 85 percent of the plant-eating cases reported, according to Goldfrank. The Poison Control Center at Children's Hospital said it receives about 65 calls a month involving plant ingestion. The amounts consumed usually are small, and Goldfrank said 90 percent of the cases reported can and should be handled over the telephone.

If a child eats part of a plant, parents may call their local poison control center (listed on the inside front covers of telephone directories) which will ask them to try to identify the plant and the amount eaten. With the exception of a few cases in which plants cause throat irritation, the usual treatment is to give the child syrup of ipecac to induce vomiting.