The Influence of Insolation on the Distribution and on the Developmental Sequence of the Flowers of the Giant Cactus of Arizona

Abstract

The Sonoran giant cactus, Carnegiea gigantea, occurs in southern Arizona and in Sonora, Mexico, down to the Yaqui River. It has a huge stem which grows to ten or twelve meters in height and which, just below its first branches, may often have a diameter of 6o or 70 centimeters. The flowers of this cactus occur in a dense crown of from I00 to 200 buds at the tip of the main stem and a similar, but smaller, crown often appears on the tip of each main branch. These flower buds arise along the 20 or 30 ribs of the stem from the areoles, or axillary buds, that were formed during the previous year (or two years). The growing point of the stem of Carnegiea is buried in a dense mass of vhite wool at the bottom of a cup.shaped depression two or three centimeters deep and from five to eight centimeters across. It is only at a distance of a centimeter or two outward from this central growing point that the separate tufts of felted hairs marking the individual areoles become distinguishable. The flower buds of Carnegiea first push through the wool of the areole in early April. About the Desert Laboratory at Tucson flower buds of Carnegiea up to 5 centimeters long were seen on April 29, I915. Opening flowers are to be found from the middle of May, or even earlier in some seasons, till the end of June. These flower buds arise in series of from one or two up to ten, or rarely to as many as fifteen, on each rib of the strongly fluted stem of this cactus. The lowest bud may be 20 or 30 areoles outward from the growing point. The buds of a rib do not arise quite simultaneously. On the contrary, it is evident from the time the flower rudiments first break through the tuft of wool on the areole (in late March or early April) that the buds near the middle of the series on each rib are commonly more advanced. This larger size of the flower buds which lie just over the outer edge of the cuplike end of the stem is evident in figure i, at the right and left above. This relation is not absolute, since there may be smaller buds, or even sterile areoles, between two larger buds (fig. i, y, z). In vigorous crowns from six to eight of the buds of a single rib may mature and bloom. A very striking peculiarity of the arrangement of the flowers of the giant cactus attracted the attention of the writer when in southern Arizona in I9I5.2 1 Botanical Contribution No. 75 from the Johns Hopkins University. 2 This study was initiated during a stay at the Desert Laboratory and was aided by a grant from the Department of Botanical Research, Carnegie Institution of Washington.