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In forage grasses, the course of seasonal production is strongly influenced by the timing of flowering and seed production, and by the extent of any winter or summer dormancy. Earlier studies on local populations of Lolium and Dactylis, covering the climatic range from the Mediterranean region to northern and central Europe, revealed a regular cline in flowering responses to temperature and photoperiod (Cooper 1960; Calder 1964a, b) and also in leaf expansion and plant dormancy at low temperatures (Cooper 1964). It was suggested that these differences resulted from the selective action of limiting climatic factors, particularly summer drought in the Mediterranean environment, and winter cold in northern and central Europe. The next step was to discover how far similar local adaptation could occur within a single climatic region. The present paper, therefore, describes some of the environmental requirements for germination and flowering, and the pattern of leaf and tiller development in contrasting light and temperature regimes, in several collections of the perennial forage grass Phalaris tuberosa, all from the Mediterranean region, but from locations differing in their seasonal distributions of precipitation and temperature, and hence in potential growing season. MATERIAL

distribution in the canopy in relation to light interception and also by the photosynthetic activity of the individual leaves. The primary aim of this paper is to investigate the effect of leaf arrangement and the optical properties of contrasting forage grass canopies on their crop growth rates and the relative importance of leaf arrangement and individual leaf photosynthesis in determining crop growth rate. A secondary aim was to use the data collected to examine the value of certain theoretical models in simulating the growth of these canopies, and hence, enabling the plant breeder to predict the consequences of selecting for particular morphological or physiological characters of the plant. The characters examined included the light distribution within the canopies, a survey of the LAI profiles, the angular distribution and optical properties of the individual leaves, and the relation between individual leaf photosynthesis and illumination.

Inflorescence development on the shoot apex and its corollary, the partition of energy in the plant between seed production and continued vegetative growth, form a developmental system of ecological and adaptive importance. A comparative study of genetic variation in the timing of this system, either within one species or between closely related species, is therefore of interest, not only from the ecological standpoint, but also as indicating possible lines of evolutionary change. The genus Lolium has a geographical distribution from central Asia, through the Mediterranean region, to the north-west coast of Europe. It contains a wide range of ecological and agronomic forms, varying from annual, self-pollinating species, such as L. temulentum and L. remotum, found as weeds in arable crops, to the biennial and perennial outbreeding species, such as L. italicum and L. perenne, which are commonly used as herbage plants. The genus therefore provides useful material for combined ecological and genetical study. Inflorescence development in Lolium follows the same general plan as in other Gramineae (Cooper, 1951, 1952), and appears to involve three main stages, similar to those postulated by Klebs (1918) for Sempervivum, each stage showing different responses to environmental conditions