Calcium Oxalate Crystals in Plants

• Star fruits contain calcium oxalate crystals. Jaime Kowal/Stockbyte/Getty Images

  Calcium oxalate crystals are generally associated with urinary conditions in animals, such as kidney stones. When they occur in plants, they are nature's way of discouraging animals and other pests from eating these plants. In fact, for some animals, such as dogs and cats and even people with food allergies or sensitives, calcium oxalate crystals can result in illness.

Jack-in-the-Pulpit (Arisaema triphyllum)

• Jack-in-the-Pulpit is a plant native to the United States and is found in moist areas in woods, such as along creeks, but also as a cultivated perennial in gardens. All parts of this plant are poisonous, though according to North Carolina State University, if the roots are properly dried or cooked, then they are safe to consume. For the rest of the plant, if eaten, it can result in severe pain in the mouth, swelling or irritation to the lips, throat and tongue.

Spinach

• Not all plants that contain calcium oxalate crystals are poisonous to humans, though they can still potentially cause crystals to form in the urine if high amounts are consumed over a long period of time. Spinach is a common vegetable that is quite high in calcium oxalate crystals. In fact, one cup of cooked spinach contains 244 mg of calcium, according to Dr. Paul Nakata, a plant biologist for the United States Department of Agriculture. However, he notes that people cannot properly digest the calcium oxalate crystals in spinach, so of the 244 mg, only 12 mg, or 5 percent, are absorbed when eaten as part of a healthy diet.

Swiss Cheese Plant (Monstera deliciosa)

• The Swiss Cheese Plant is commonly used as a house plant, and therefore could pose a danger to pets or young children. This tropical plant features large leaves with unusual holes, hence the name "Swiss Cheese Plant." It can produce small fruits that are described as "pineapple-like" by North Carolina State University. When any part of this plant is ingested, it can result in nausea, vomiting or diarrhea, as well as irritation to the oral cavity of pets and humans.

Star Fruit (Averrhoa carambola)

• The star fruit, a native plant to the country of Indonesia, is actually safe to eat. The tart flavor of the star fruit is the result of an oxalic acid that forms when saliva is combined with the calcium oxalate crystals in the fruit. Though it is safe to eat, according to the University of Washington, the crystals can reform in urine, especially if a large number of foods with calcium oxalate crystals are eaten over an extended period of time.

Taro (Colocasia esculenta)

• The taro, which is also referred to as a yam, according to Oregon State University, has a high concentration of calcium oxalate crystals within its skin. When eating taro, it is important to peel the skin as well as a few layers of the taro to reduce the chance of eating any of the crystals, which can be irritating. This is because the crystals can "become lodged in the mucous membranes of the mouth and throat," according to the University of California.

Calla Lily (Zantedeschia aethiopica)

• The calla lily, also known as the florist's calla, is a popular house plant, wedding flower and, in the right regions, garden plant. However, like many members of the Araceae family, the calla lily is poisonous and should be kept out of reach of young children and pets. The calcium oxalate crystals in this plant cause irritation to the oral cavity.