-STICS (Simulateur mulTJdiscplinaire pour les Cultures Standard) is a crop model constructed as a simulation tool capable of working under agricultural conditions. Outputs comprise the production (amount and quality) and the environment. Inputs take into account the climate, the soi1 and the cropping system. STICS is presented as a model exhibiting the following qualities: robustness, an easy access to inputs and an uncomplicated f~~t u r e evolution thanks to a modular (easy adaptation to various types of plant) nature and generic. However, STICS is not an entirely new model since most parts use classic formalisms or stem from existing models. The main simulated processes are the growth, the development of the crop and the water and nitrogenous balance of the soil-crop system. The seven modules of STICSdevelopment, shoot growth, yield components, root growth, water balance, thermal environment and nitrogen balanceare presented in tum with a discussion about the theoretical choices in comparison to other models. These choices should render the model capable of exhibiting the announced qualities in classic environmental contexts. However, because some processes (e.g. ammoniac volatilization, clrought resistance, etc.) are not taken into account, the use of STICS is presently limited to several cropping systems. (

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Examination of the time courses, expressed in terms of accumulated temperature over O"C, of leaf appearance of fifteen sowings of winter wheat, including five cultivars, over three seasons at Auchincruive, revealed several examples of bi-linear rather than linear responses. Stepwise linear regression analysis, varying the temperature threshold by intervals of 1 "C, showed that a linear relationship could be obtained for each sowing, but the appropriateness of this analysis is questioned. An alternative interpretation, that such changes in rate do have biological rather than purely statistical significance, is supported by the fact that, when they occurred, they coincided with the change in rate of spikelet initiation by the stem apex. Furthermore, similar relationships between leaf appearance and reproductive development have been demonstrated for perennial ryegrasses. Analysis of each sowing also revealed the existence of sub-populations of plants with different rates of leaf appearance and final leaf numbers, but with synchronous phenologies. These findings are discussed in relation to the coordination of the development of apex and leaves, and the environmental cues required to ensure this coordination.

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