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Summary 1Germination and seedling performance may set the limits for plant distributions, particularly in stressful habitats. Stressful conditions at these early stages may be avoided by opportunistic germination and growth, or may be tolerated. Many epiphytic plants are frequently exposed to severe drought. Adult epiphytes endure such dry periods in various ways, but little is known about strategies employed during germination and early life. 2Epiphyte seedlings could show either opportunistic fast growth to quickly attain the benefits of being larger or inherently slow growth and early drought tolerance. Here we address the question: which of these strategies characterizes the early life stages of the epiphytic bromeliad Tillandsia flexuosa, a species typical for dry tropical habitats? 3We studied growth and drought tolerance of germinating seeds, of the emerging seedlings and of 2-month-old seedlings under controlled conditions. Additionally, we studied drought hardening in 6-month-old seedlings. 4Germination of T. flexuosa was reduced by intermittent dry periods. However, compared to the congeneric T. fasciculata, which typically occurs in wetter habitats, the depression of germination by drought was small. Seedling growth was fastest at intermediate moisture levels: both prolonged drought and continuous moisture depressed growth. Prolonged drought had a less negative effect on drought-hardened seedlings than on previously well-watered seedlings. After a 3-week drought treatment the previously well-watered seedlings had lost their growth advantage entirely. Had drought continued, they would have probably been starved, indicated by the low level of their non-structural carbohydrate pool. 5Tillandsia flexuosa employs a stress-tolerance strategy both during germination and during the seedling stage. In its epiphytic habitat this strategy is clearly adaptive, considering the predictable briefness of moisture availability throughout the year and the low competition pressure that allows the very slow growth typically seen in adults and seedlings. These conditions characterize not only the dry-forest habitat of T. flexuosa, but all exposed epiphytic growing sites, so we expect this early stress-tolerance to be common among epiphytes in general. Still, a lower stress tolerance in species from wetter habitats may at least partly explain why T. flexuosa shares its dry-forest habitat with so few other vascular epiphytes.

The leaves of plants show an almost infinite variety of form although most leaves are essentially organs utilized for the same function, photosynthesis. Analyses of the diversity of plant leaf form have revealed the presence of one major trend in the variation, i.e. from large, relatively thin and soft leaves to small, thick and tough ones (Givnish 1987; Reich, Walters & Ellsworth 1992; Turner 1994), although there is a great deal of scatter about any regression line. The subject of this paper is the leaves at latter end of the spectrum which I will call sclerophylls, using the term in its literal, descriptive sense of 'hard leaves'. Soft leaves will be called malacophylls. The sclerophyll is the predominant leaf form in vegetation occurring over a wide geographic and climatic range. This has been recognized for at least the last century (Seddon 1974) but the functional significance of sclerophylly remains controversial (Cody & Mooney 1978; Grubb 1986). Hypotheses concerning the significance of sclerophylly fall into three categories. It has been argued that sclerophylls are primarily adapted for water conservation, nutrient conservation or the prevention of damage. I believe that critical examination of the data available indicates that the anatomical features typical of sclerophylls serve a fundamentally protective function. The reasons for this statement will be discussed after reviewing the characteristics of sclerophylls.

The reproductive characteristics of 27 species of shrubs of coastal macchia from southern Italy were investigated. There are two main groups of species, anemophilous and entomophilous. Amongst the entomophilous species are two diverging trends in reproductive characteristics, towards either a group of dioecious, small-flowered, fleshy-fruited, few- to one-seeded, vegetatively spreading species or to an hermaphrodite, large-flowered, dry-fruited, many-seeded, non-vegetatively spreading group. The relationships between breeding system and both fruit type and seed number were examined for 129 shrubs of the maquis and 111 of the sclerophyllous forest of the Mediterranean (...)

1. A root-segment model was combined with a soil water model to predict water uptake by roots of Agave deserti and Ferocactus acanthodes of four different lengths at 10 orientations from horizontal to vertical after various simulated rainfall events. 2. The soil was divided into 15 strata with specific water potentials (Ψ soil ) and temperatures; each root segment had specific values of radial hydraulic conductivity from the root surface to the xylem (L R ) and of axial hydraulic conductivity along the xylem (K h ). For roots of the monocotyledonous A. deserti, L R decreased with distance from the root tip up to 80 cm and K h became constant and maximal after 40 cm, favouring water uptake from the distal (younger) portions

Carbon isotope discrimination (4) has been shown to be a valuable tool for long-term estimates of water-use efficiency, because this parameter is influenced by the ratio of intercellular to ambient CO 2 levels, which is in turn related to water-use efficiency. The objective of this study was to examine variation in 4-values of field materials and the extent to which changes in 4 might be interpreted as changes in water-use efficiency. On-line discrimination observations with four desert species confirmed theoretically expected relationships between 4 and the ratio of intercellular to ambient CO 2 concentrations, a prerequisite to applying 4-values to water-use efficiency (...)

Seasonal plant and tissue-water relations were examined in fully expanded twigs of three sclerophyllous, evergreen Quercus species (Q. chapmanii, Q. inopina and Q. myrtifolia) in south-central Florida, USA. In these species we observed plateau effects when obtaining pressure-volume curves. Such effects may cause significant interpretational errors in osmotic potentials and elastic moduli, and be confounded with seasonal changes in these parameters. New leaves are produced and mature in late-March to early-May of each year (...)