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.