Seeds of Evolution: The Botanical Triumph of Land Plants

Approximately 450 million years ago, the first plants, ancestors to modern moss and ferns, established themselves on land, reigning over the vegetative realm for more than 100 million years. These early ferns lacked seeds and relied on water for successful sexual reproduction. The male gametophyte released sperm that had to navigate through a watery medium to meet and fertilize an egg, forming a zygote.

Seed plants enable pollen grains to transport male sperm over extended distances, even in challenging weather conditions, to facilitate the fertilization of ovules. The seeds in question originate from "lucky bean" trees (Erythrina lysistemon), which belong to the pea family. These trees are often cultivated in parks and gardens, and they are associated with mystical and therapeutic qualities.

Around 350 million years ago, seed plants emerged, marking one of Earth's most significant biological milestones. They have since remained the dominant and most recognizable form of vegetation. Seeds and pollen were pivotal in this transition, liberating plants from their dependence on water for reproduction and granting them the ability to adapt to arid conditions and the harsh ultraviolet radiation of sunlight.

Unlike seedless plants that produce a single type of spore leading to a bisexual gametophyte, seed plants evolved to produce two kinds of spores: microspores, which give rise to multiple male gametophytes, and megaspores, resulting in a single female gametophyte. The female gametophyte, enclosed by a protective coat, forms an immature seed known as an ovule. Pollen grains serve as male gametophytes, each housing sperm within a protective coating that prevents desiccation, provides mechanical resilience, and facilitates long-distance dispersal. Unlike sperm in seedless plants, which must actively swim to the ovule, seed plant sperm are transported passively by air currents.

The transfer of pollen grains to the ovule-containing part of the plant is termed pollination. Once the sperm fertilizes the egg cell within an ovule, the ovule develops into an embryo, maturing into a seed. Seeds offer protection, nourishment, and the ability for the embryo to remain in a dormant state for extended periods, awaiting favorable environmental conditions for germination.

Seed plants, or spermatophytes, fall into two major categories: gymnosperms (literally "naked" seeds), encompassing conifers, and angiosperms, also known as flowering plants, which represent a staggering 250,000 species—accounting for approximately 90 percent of the plant kingdom.