The _rigidissimus_, or Rainbow Cactus, another of the cylindrically formed cacti, is easily identified by the noticeably stiff spines, which are very numerous. The stems grow to about fifteen inches tall, four inches or less in diameter with a rounded-off top, singly, or branched above. The cylinder is composed of about twenty-four ridges well covered with very sharp radials, but with no central spines, a characteristic uncommon in the Hedgehog group. There are great numbers of radially placed spines, closely pressed to the sides of the stem. They are arranged in two comb-like groups, one on each side of the areolas, and interlock with thorns of adjacent spine clusters. This makes a dense spine layer over the entire surface of the plant. These spines are less than a half-inch long, and have swollen bases. They form in many colored zones, or whitish, yellow, rose-purple to maroon-purple bands about the plant. The flowers are about three inches long and as wide when full open, and have some forty petals and thirty sepals. The petals have pointed tips and are bright rose-purple with yellowish bases. The fruit is covered with many spines, is about the size of a strawberry, fleshy and sweet. The _rigidissimus_ invariably grows along the rock ridges and rocky foothill slopes at altitudes of thirty-five hundred to fifty-five hundred feet, the roots growing among the rocks. Its distinguishing characteristic is the many colored bands of the spines around the plant, suggesting a rainbow. How to grow Rainbow Cacti are not injured in zero temperatures out of doors, but with colder weather than this they require some protection. They may be grown in cool, dry, sunny conservatories. The Rainbow is one of the most difficult of the cacti to grow under cultivation. Unless planted in rocky soil, preferably early in spring, it will grow for only a year or two. Young plants transplant more readily than older ones, and neither the roots nor the stems should be injured in digging. Give enough water to keep the soil moist during the growing season. Plants may be grown from seed in moist sandy soil in part shade or diffused sunlight. When mature plants are used and transplanted in early spring they blossom the same season. [Illustration: RAINBOW CACTUS (Echinocereus rigidissimus)] A popular beauty, named from the brilliantly colored spines arranged around the plant in many-hued bands or “zones.” Called by the Mexicans Cabecita Del Viejo. Short Spined Strawberry Cactus (_Echinocereus Bonkeræ_) (Named for Frances Bonker, one of the authors of this book) How to identify and how it grows