The Natural Resources Conservation Service under the United States Department of Agriculture maintains a database on plants in the US. Plants native to Missouri can be found in the following link .

Since there are very many plants native to Missouri, the following are some I found interesting:
*Crataegus punctata*, or dotted hawthorn, is considered by some the state flower of Missouri. However, the law only specifies the state flower to be a "red haw" or "wild haw" without mentioning any particular species.
*Crataegus mollis*, or red hawthorn, is considered by the Missouri Department of Conservation to be the state flower. Hawthorns can be either scrub or small trees. They have thorny branches and give small pome fruits.
*Elymus canadensis*, or Canada wild rye, is native through out most of North America, including Missouri. They have symbolic relationship with a number of fungi. It has been though that the process helps the Canada wild rye to grow in dry environment. It is planted in reclaimed mines due to its adaptability to metal-rich soils.
*Wisteria frutescens*, or American wisteria, is native throughout southeastern United States, including Missouri. It has beautiful purple flowers. However, it contains a toxin called Wisterin like all other Wisteria.