ABSTRACT
The supposed endosymbiotic origin of plastids makes it particularly appropriate to consider the cell as a habitat. Successive stages of plastid development can be regarded as a parallel to successive stages of colonization; productivity increases to a peak and then declines during the life of a chloroplast, just as it does during the life of an ecosystem. The evolution of land plants has been accompanied by a conspicuous increase in the range and complexity of secondary plant metabolites, and the plastids have been implicated in the synthesis of many of these or of their associated carbohydrates. Plastid dedifferentiation may well have arisen in the first place as a response by the cell to stress conditions irregularly imposed by the unpredictable environment. The proplastid may well have been the original key to the development of new and functionally more flexible forms of plastid in specialized cells which represent adaptations to the land environment.
