Evolutionary and ecological aspects of recalcitrant seed biology

There is a substantial literature on the basic physiology and response to d esiccation of recalcitrant seeds, but little is known about their ecology a nd even less of their evolutionary status. It is difficult to assess the re sponse of early land plants to dehydration, but it is likely that desiccati on tolerance in vegetative tissue arose concomitantly with the invasion of the land. Similarly, from the fossil record it is not possible to assess th e desiccation response of early seeds, and furthermore, it is difficult to see phylogenetic relationships among species producing recalcitrant seeds. A consideration of the available evidence, however, suggests that the first seeds were desiccation-sensitive, but tolerance evolved early and probably a number of times, independently. The desiccation sensitivity and short li fe span (generally shorter than the interval between flowering) of recalcit rant seeds have implications in terms of regeneration ecology. A long-term soil seed bank as such does not exist; rather the seeds germinate and form a seedling bank. However, there is a wide range in post-shedding physiology among recalcitrant seed species, and although species producing recalcitra nt seeds are common in the humid tropics, they do occur in habitats with mo re marked seasonal variation. Here regeneration strategies may be more spec ialized.