34.4:
Introduction to Seed Plants
The most successful major group – the seed plants – includes almost 400,000 species. Seed plants themselves are divided into two groups – the first being the gymnosperms, which are predominantly conifers, but also includes plants like cycads or ginkgos.
The second group is the angiosperms, which includes over 90 percent of known plant species, including magnoliids and other flowering trees, all fruits, and flowering plants.
As seed plants, both gymnosperms and angiosperms share several key features. First, their life cycles are dominated by the sporophyte stage. Second, they have microscopic gametophytes which form haploid gametes – including female gametes called ovules which are typically housed in structures like cones or ovaries to protect and nourish them.
The male counterpart gametes called pollen grains are formed in separate structures, and are easily dispersed by environmental methods like wind, or animals, to hopefully encounter and fertilize the female gametes.
Finally, once fertilized, these now diploid structures form seeds – structures which contain nourishment for the future seedling.
Here, the differences between the lineages begins – while gymnosperms’ seeds are typically housed in cones or on scales which are open to the air or animals for dispersal, angiosperms may produce fruits surrounding or attached to their seeds.
Fruits contain one or more seeds, and facilitate seed dispersal. For example, animals may eat the fruit and then leave the area, before passing the seeds which often remain intact.
Alternatively, the fruits can help the seeds to float, fly, or hitch a ride on an animal to their final destinations.
Whatever the method of dispersal, when the seeds reach their germination spot, another classification distinguishes the different seed plant lineages – cotyledon number. Cotyledons are a part of the seed that forms the embryonic leaf or leaves upon germination.
Gymnosperms are referred to as multi-cotyledenous and will most typically have anywhere from 8 to 20 plus of these embryonic leaves, which may grow in a whorl shape around the embryonic stem.
Most angiosperms on the other hand have either one or two cotyledons, and are broadly categorized as monocots or eudicots based on this number and this difference is seen right from the initial seed.
Examples of eudicots include oaks and roses, while monocots include grasses, orchids, and corn.
Aside from cotyledons, several other features distinguish monocots from eudicots. For example, the arrangement of roots, which are typically tap root or fibrous root systems, or the vascular tissue in stems, which occurs in a ringed arrangement in eudicots and is scattered in monocots.
Additionally, leaf veins form a net-like structure in eudicots and a parallel arrangement in monocots and flower organ development occurs in segments of four to five in eudicots, versus threes in monocots.
Overall, these many and varied adaptations of seed plants have allowed them to become the dominant vegetation on Earth.
34.4:
Introduction to Seed Plants
Most plants are seed plants—characterized by seeds, pollen, and reduced gametophytes. Seed plants include gymnosperms and angiosperms.
Gymnosperms—cycads, ginkgo biloba, gnetophytes, and conifers—typically form cones. The pollen cones contain male gametophytes. The ovulate cones contain female gametophytes and form exposed seeds when fertilized.
Angiosperms, the most diverse and ubiquitous group of land plants, form flowers, and fruit. Like the cones of gymnosperms, the flowers and fruit of angiosperms enable sexual reproduction.
Flowers facilitate pollen dispersal. The fertile flower structures—stamens and carpels—contain male and female gametophytes, respectively. Fruits facilitate seed dispersal, often forming after flowers have released pollen. As seeds develop from a flower’s fertilized ovules, the ovary wall thickens, forming a fruit containing seeds.
Angiosperms were historically categorized as monocots or dicots based on their number of cotyledons—or seed leaves. However, based on genetic evidence, most species classically considered dicots are now called eudicots. Legumes (e.g., beans) and most well-known flowering trees (e.g., oaks) are eudicots.
The other former dicots belong to one of four small lineages. Three of these—Amborella, water lilies, and star anise and its relatives—are considered basal angiosperms due to their early divergence from ancestral angiosperms. The fourth group—the magnoliids—contains thousands of species, including magnolias.
Examples of monocots include orchids, grasses, palms, corn, rice, and wheat. Aside from cotyledon number, other characteristics distinguish monocots from eudicots. Leaf veins are typically parallel in monocots and netlike in eudicots. In stems, the vascular tissue is often scattered in monocots and ring-like in eudicots. Unlike eudicots, monocots generally lack a primary root. Pollen grains typically have one opening in monocots and three openings in eudicots. Finally, flower organs are often found in multiples of three in monocots and multiples of four or five in eudicots.
Suggested Reading
Coen, Olivier, and Enrico Magnani. 2018. “Seed Coat Thickness in the Evolution of Angiosperms.” Cellular and Molecular Life Sciences 75 (14): 2509–18. [Source]
Linkies, Ada, Kai Graeber, Charles Knight, and Gerhard Leubner-Metzger. 2010. “The Evolution of Seeds.” New Phytologist 186 (4): 817–31. [Source]