Chapter 14: The Development of Seeds
Superficially, the production of seeds (Fig. 1-2) resembles the production of offspring in familiar animals: inside a diploid parent there develops a member of the ‘next generation’, which is nurtured inside its parent during the critical early stages of development and then is deposited outside its parent to finish its life. But appreciate that all plants exhibit an alternation of generations, so if a diploid (sporophyte) plant produces a new diploid (sporophyte) plant in a seed, one must account for the haploid gametophyte generation that had to come in between the two sporophyte generations. And one must also appreciate that seeds are NOT a substitute for spores, in fact, spores are critical to the production of seeds. The appearance of seeds (both in the sense of evolution and in the sense of development) is a complex story, one that involves the pattern of ‘alternation of generations’ shown in all plants. In light of this pattern, seeds represent a ‘babushka’ (Russian doll) with multiple generations found inside each other. An appreciation of this ‘generation within a generation’ is essential in understanding ‘how seeds came to be’ both evolutionarily and developmentally.
While it was long assumed that a structure as complex as seeds evolved once, many now feel that seeds evolved multiple times. Seeds therefore may represent an example of convergent evolution, where multiple lines have converged on a common feature. Whether or not this is actually the case, we can cite several features that allowed seeds to evolve and some of these features are exhibited in groups that do not produce seeds. Central to the appearance of seeds, in both a developmental and evolutionary sense, is the appearance of ovules, dynamic entities whose composition changes, ultimately ending up as a seed. In this chapter, we consider the transformations in the life cycle of plants that allowed for the development of seeds. In the next chapter, we consider the specific structures and patterns seen in conifers and flowering plants. Although we are focused on the seed, we will also consider a companion entity that is essential for the development of seeds: the pollen grain, which we will see is a miniaturized mobile, male gametophyte.
TOPICS
- Seed Structure
- Reduction
- Retention
- Arrested Development
- Provisioning
- Packaging
Seed Structure
A seed consists of three components: an embryonic sporophyte plant, a tissue that provides nutrition to that embryo, and a ‘seed coat’, the container tissue in which the embryo and nutritive tissue develop. The embryonic plant is diploid and it develops from a zygote formed by the union of egg and sperm. The seed coat is also diploid and it also is derived from a sporophyte plant, but it is an earlier sporophyte generation than the embryo. In both a temporal and also in a physical sense, a seed is a generation ‘babushka, a Russian doll’, with ‘nested’ generations. There are two sporophyte generat