Big Bend has only three types of mammillaria cacti, a genus much more common in Mexico. Mammillaria heyderi is by far the largest, forming a large, unbranched stem that grows close to the ground, making the plant somewhat difficult to locate, despite being quite widespread at all locations below 5,300 feet elevation. Flowers are pale yellow in color, appearing in early summer in a ring near the apex. The other two mammillaria species in the park (m. lasiacantha and m.pottsii) have smaller stems, covered in short, thick white spines, and form clusters.
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