A year ago, my night-blooming cereus (a cactus) was just one leaf. In July, when it's supposed to bloom, a dozen of its 6- to 12-inch long scalloped leaves grew out of each other, and I was excitedly waiting to see (and smell!) my first flower.

Then I ran an ozone generator to get rid of mold in our old house. Now I learn ozone interferes with photosynthesis, resulting in visible leaf injury, growth and yield reductions, and greater sensitivity to stress. Since I knew the gas is harmful to humans, I ran the generator at night, and an exhaust fan first thing in the morning. At night the stomata (breathing pores) of most plants close.

Except for epiphytes (like staghorn fern, bromeliads, and many orchids), crassulas (including jade plant and kalanchoe), and some cycads (palms) ... none of which I have; but several plants I do: Agaves, like my snake plant, on which for the first time in the plant's fifteen-year life, one of the three-foot, sword-shaped leaves has fallen, limp and gray. Other succulents, including two I have in the lily family: my "zebra cactus" and my aloe veras. And cacti: the cereus didn't bloom.

I ozoned most in the kitchen, where I grow several aloe veras. To heal hurt skin I open a piece of leaf and tape it to the spot, pulpy side down. The plants haven't been growing as fast as usual. They still heal my skin, but the pulp is more watery than it should be. I hope they recover; I use them a lot in the winter.

Plants that normally open stomata at night, don't open them at all if they're in drought stress. Instead of taking in carbon dioxide needed for photosynthesis, they just recycle the CO2 produced from their own oxidative respiration. I usually let my houseplants get rather dry in summer — partly from laziness, partly to keep mold down. In fact I've chosen most for their drought tolerance.

After July, the cereus grew a two-foot stem with dozens of aerial roots, telling me it's thirsty. Maybe drought protected it from ozone, but kept it from flowering? As usual, my Christmas cacti are setting flower buds since I started watering them more this fall.

Mold gives me headaches; I hate it. I'll move plants, I'll ... hmm: could I use ozone at night to kill damping-off disease when I start seeds indoors?

• Night-blooming cereus - pictures of the flower I haven't seen yet. Although I found an amazing number of related cacti all with this common name, this is the one with leaves that look like mine.
• Effect of ozone on plants - on the site of Agriculture and Agri-Food Canada
• Epiphytes - adaptations to an aerial habitat - (Royal Botanic Gardens, Kew: Information Sheets)
• Staghorn Fern by Dr. Bob Black, Consumer Horticultural Specialist at the University of Florida's Environmental Horticulture site
• Crassula ovata - (jade plant). A good simple description here of Crassulacean Acid Metabolism (see Types of Photosynthesis, below.)
• Mother in law's Tongue - Sanseviera trifasciata (aka snake plant)
• Haworthia attenuata var. attenuata - Zebra cactus (Liliaceae). Hmm, after reading more about this plant I think I was in error to say it normally closes stomata at night; rather it's one of the plants which will close stomata indefinitely in drought (see Hanscom et al, below.)
• Cactus - Gardening Austrialia Fact Sheet
• Aloe Vera - at The Dirt Doctor. Other interesting aloes here, too.
• Types of Photosynthesis - The names of the three are C3,C4 and CAM (Crassulacean Acid Metabolism, so named because it was first found in Crassulaceae.)
• Responses of Succulents to Plant Water Stress - by Zac Hanscom, III and Irwin P. Ting. Peperomia (though not normally classed CAM) and simlarly, Cacti, close stomata completely in drought stress.
• Epiphytic Cacti - Schlumbergera
• Effects of Ozone and Acidic Deposition on Gas Exchange Responses in Ponderosa Pine Epiphyllum oxypetalum - on the site of Pima Community College, Tucson, Arizona. Research notes from the California Environmental Protection Agency. Seedling pines suffered less from ozone than mature branches.
  
  Maybe other seedlings will have similar resistance. But so far, I haven't figured out how to exhaust the ozone in the morning, without freezing the seedlings — as well as the rest of the house, since the fan I used in the summer was the whole-house fan in the attic.