Anatomy and water relations were studied for the desert fern Notholaena parryi, as well as six other ferns representing three different orders which occupied xeric as well as mesic habitats. Tracheid number and diameter, and total xylem cross sectional area increased during leaf development for N. parryi; the whole plant conductance (volume flow of water through a stipe divided by the rhizome‐to‐leaf water potential drop) increased but tended to level off as the leaves matured. The reported occurrences of very steep water potential gradients (about 25 MPa m–1) in stipes of N. parryi were confirmed. The ferns with the highest whole plant conductances (Alsophila australis, Botrychium dissectum, and Adiantum capillus‐veneris) had the largest or greatest number of tracheids. Numerous tracheids in Botrychium dissectum offset a low tracheary conductivity, whereas Marsilea vestita had few tracheids resulting in a low whole plant conductance. Whole plant conductances for the ferns were 2 to 3 orders of magnitude less than those generally observed for angiosperms and 6 orders less than for gymnosperms. However, the relative conductivity (whole plant conductance times stipe xylem length divided by xylem area) was only 5‐ to 10‐fold less than for angiosperms and about the same as for the gymnosperms. Stipe water relations in these ferns are discussed in relation to the evolution of xylem anatomy.

Both field measurements and a computer model were used to study the interception of photosynthetically active radiation (PAR) by Agave deserti (Engelm.), a desert CAM plant with a basal rosette of massive opaque leaves. PAR interception was determined in the winter and the summer for upper and lower leaf surfaces on a plant with about 60 leaves. Total daily PAR on the leaf surfaces was approximately 10 mol m‐2 for a winter day and 20 mol m‐2 for a summer day. For a PAR of 15 mol m‐2, the nocturnal increase in acidity was about 0.6 mol m‐2 for both leaf surfaces and various leaf orientations, except for the oldest most horizontal leaves where the increase was less than half as large. The acidity increase measured in the field was 90% saturated at 25 mol m‐2. Thus, daytime PAR in the desert is often limiting for the nocturnal acidity increase, especially for the lower leaf surfaces. Simulated tilting of the plant by 55° so that the vertical axis pointed to the sun at solar noon on a winter day increased the PAR incident on the upper surfaces of the leaves, but did not affect the total nocturnal increase in acidity by the whole plant. Although simulated removal of alternate leaves increased the PAR per unit leaf area for the remaining leaves, it reduced the total increase in nocturnal acidity of the whole plant by 31%. PAR interception by plants on slopes facing steeply north, east, or west was substantially reduced compared to the horizontal. Thus, the model proved to be quite useful for quantifying the relation between leaf orientation, PAR interception, and nocturnal increases in acidity by A. deserti, and it indicated that the lower frequency of plants on north‐ compared to south‐facing slopes was due to PAR limitations.

A simulation model has been developed to describe the thermal relations of individuals of an important group of desert succulents, the agaves, similar to previous modeling efforts on cacti. The model utilizes an energy budget approach to evaluate the effect of various morphological and microclimatic parameters on plant temperature and water loss. For an Agave deserti 0.5 m tall with a basal rosette of 60 leaves, the predicted surface temperatures differed by an average of only about 1°C from those measured in the field in the western Sonoran Desert. Stimulations indicated that leaf and stem temperatures as well as plant water loss were especially sensitive to changes in air temperature. Nocturnal stomatal opening reduced leaf surface temperatures by only 1.4°C. Increasing the shortwave absorptance from the measured value of 0.45 to 0.80 caused the maximum leaf surface temperature to increase 8°C. Stimulated increases in plant size markedly reduced the diurnal range of stem tissue temperatures, and simulated decreases in size reduced the diurnal range in leaf surface temperatures. The small stature of A. utahensis would result in higher minimum leaf temperature and may account for its survival at a cold site in Nevada. Water loss per plant varied approximately as the square of the linear dimensions, which may help explain the decreasing height of agave species with increasing aridity from central Mexico northward. Thermal buffering of the meristematic region in the stem apex by the surrounding massive leaves may also be quite important for the growth and distribution of agaves.

Anatomy and water relations were studied for the desert fern Notholaena parryi, as well as six other ferns representing three different orders which occupied xeric as well as mesic habitats. Tracheid number and diameter, and total xylem cross sectional area increased during leaf development for N. parryi; the whole plant conductance (volume flow of water through a stipe divided by the rhizome‐to‐leaf water potential drop) increased but tended to level off as the leaves matured. The reported occurrences of very steep water potential gradients (about 25 MPa m–1) in stipes of N. parryi were confirmed. The ferns with the highest whole plant conductances (Alsophila australis, Botrychium dissectum, and Adiantum capillus‐veneris) had the largest or greatest number of tracheids. Numerous tracheids in Botrychium dissectum offset a low tracheary conductivity, whereas Marsilea vestita had few tracheids resulting in a low whole plant conductance. Whole plant conductances for the ferns were 2 to 3 orders of magnitude less than those generally observed for angiosperms and 6 orders less than for gymnosperms. However, the relative conductivity (whole plant conductance times stipe xylem length divided by xylem area) was only 5‐ to 10‐fold less than for angiosperms and about the same as for the gymnosperms. Stipe water relations in these ferns are discussed in relation to the evolution of xylem anatomy.

The fishbone potential of composite particles simulates the Pauli effect by nonlocal terms. We determine the α − α fishbone potential by simultaneously fitting to two-α resonance energies, experimental phase shifts and three-α binding energies. We found that essentially a simple gaussian can provide a good description of two-α and three-α experimental data without invoking three-body potentials.