Northward orientation of Copiapoa stems and apex warming

North in the southern hemisphere

The study sites are in Chile, in the southern hemisphere. A north-facing apex points towards the equator and a substantial part of the Sun's daily path. Ehleringer and colleagues described this phenomenon in 1980 for three taxa recognised at the time. [Ehleringer et al. 1980, pp. 63–64]

The current names of the plants studied are C. cinerea, C. cinerea subsp. columna-alba (in the paper, C. columna-alba) and C. gigantea (in the paper, C. haseltoniana). Retaining these links allows tables and figures in the older publication to be read correctly. [Larridon et al. 2018][Ehleringer et al. 1980]

Two different angles

The direction of inclination is described by azimuth, the position relative to compass directions. The angle of the stem axis to the horizontal describes its degree of elevation. In a sample of 100 C. cinerea subsp. columna-alba plants, 93 deviated from north by no more than 20°, and all fell within 40°. The table gives the second measurement, the angle to the horizontal. [Ehleringer et al. 1980, Table 1]

A warm apex in cool air

Measurements on 12 September 1978 in Pan de Azúcar included air temperature, the woolly apex and tissues beneath the surface. Air temperature was around 13–19.9°C. Around 30–40°C was recorded in the apical stem zone for much of the day. The difference from the surroundings reached about 15–20°C. [Ehleringer et al. 1980, pp. 65–66]

The apex contains the meristem, tissue that produces new stem structures, and the flower development zone. Its solar exposure and woolly covering create different energy exchange conditions from those on the plant's sides. Radiation absorption measurements also compared hairs, the waxy surface and the surface after wax removal. [Ehleringer et al. 1980, pp. 64–66]

Modelling result

The authors compared a north-facing stem model with the average for four orientations. For the apical surface, the model indicated about twice as much incoming radiation in winter and about 40% more in spring. The stem sides received less during part of late summer and autumn. These are geometric calculations for the assumed shape and conditions, complementing field measurements. [Ehleringer et al. 1980, pp. 65–67]

This energy distribution favours warming of the growth zone in a cool coastal climate while limiting the load on part of the lateral surface. In the study, the favourable model angle was close to about 50°, corresponding to the mean for the columna-alba population. The other populations had their own angle distributions, shown in the table. [Ehleringer et al. 1980]

From orientation to the growth mechanism

The paper documents stem orientation and its energy effects. How asymmetric growth establishes this direction during development was not directly investigated. The physiological explanation obtained in the publication concerns primarily the radiation balance and temperature. The phenomenon relates to the geographical location and form of the plants measured. [Ehleringer et al. 1980]

The traits of the subspecies studied can be compared in the profile of C. cinerea, while a description of plants formerly called C. haseltoniana is provided under C. gigantea. [Larridon et al. 2018]