A Study of the External Surface of Opuntia Imbricata (Haworth) DC

Abstract

A comparative study was made of the subterranean and aerial por- tions of 0. imbricata. This plant had a fibrous root system which branched out into the quaternary division. Roots of both tertiary and quaternary divisions were fairly well covered with hairs. The surface area of the tops exceeded the subterranean sur- face by about 3:1. Stomates in the epidermal surface of the canes numbered 11 per sq. mm. The purpose of this investigation was to gain a better understanding of a very successful species living in an arid environment. Most of the work on desert plants has been concentrated on aerial portions and the various modifications of these plants to endure the dessicating winds, intense heat, sparse rainfall, and infertile soils characteristic of these regions. In 1911 Cannon wrote extensively of the rooting habits of desert plants in Arizona. His descriptions were confined mainly to the major roots and did not include detailed surface relationships. Since that time several papers have appeared with data on the root systems of various plants. Much of Weaver's work (1925; 1926) dealt with the root systems of crop and field plants and also grasses of the Great Plains. Included in the 1925 investigation was an estimate that root surface area exceeded the top by 35 per cent. Observations by other investigators have resulted in similar opinions, although no de- tailed study of complete surface relationships were available until Dittmer (1937) described the external surfaces of a winter rye plant. Earlier studies by Kiesselbach (1910) reported the dry weight of the tops of corn to be 7.2 times that of the roots and Pavlychenko (1937) described the entire root system of crested wheat, but complete studies showing the surface relationships of individual plants have been few. Usually one worker has described the root system of a plant or a species and another described the tops. Even though the figures may not have been of the same species and certainly not the same plant, a third investigator often attempted to correlate these results. In this paper the writer presents complete figures for a single xeric plant.