The vast majority of plants that we observe and utilize possess seeds and seed
The vast majority of plants that we observe and utilize possess seeds and seed plants dominate most terrestrial habitats. The last chapter described in general terms what seeds are and what modifications in the general plant life cycle of alternation of generations had to occur in order for seeds to appear. This chapter fills in some of the details for the five groups of seed plants:
- flowering plants, with 250,000 species, by far the most diverse and ubiquitous plant group
- conifers, although with only 1000 species, they are very commonly encountered and ecologically and economically important
- cycads, a small group of around 300 species with limited distribution and importance
- gnetophytes, a small group of only three genera and around 100 species
- Ginkgo, a single species that survives only where cultivated
In addition to these five extant groups, there are several groups of seed plants well represented in fossils but no longer present. These extinct species are sometimes lumped as ‘seed ferns,’ but both the lumping and the designation as a group are not thought to be accurate: they are not closely aligned with ferns and they probably represent an artificial (polyphyletic) grouping. Most workers believe that seeds evolved more than once and therefore that there should be no phylogenetic entity corresponding to either seed plants or to gymnosperms (i.e., seed plants lacking flowers) although these categories do persist. We will consider the details of the life cycle, in particular the form of the female gametophyte and the mechanisms associated with pollination and fertilization, for the five groups of extant seed plants.
TOPICS
- Conifer seed development
- Seed development in other gymnosperms
- Seed development in angiosperms
- Flowers
- Floral modifications
- Fruits
Conifers
Pines are the most commonly seen conifers and the group that will be described below but the basic pattern holds for all of the group. The plants that are recognized as pines are diploid, sporophyte plants. All conifers, including pines, are heterosporous and produce two kinds of spores, both on the same tree. The sites of spore production are the cones. The cones that most people recognize as ‘pine cones’ are female pine cones. These are not only the site of megaspore production but also the site of: megaspore germination that forms a gametophyte, egg production by that gametophyte, egg fertilization, and ultimately seed development. All these events take place in a location described as an ovule. These processes generally take multiple years and the structures one usually recognizes as pine cones have been living and developing over a time of two years or more, with many of the significant events occurring when the cone is much smaller and not as easily observed (Fig 1). A female cone consists of an axis (stem) bearing scales subtended by bracts, with the scales thought to be derivatives of modified branches. Upon the upper surface of the scale are the ovules, th