植物多样性概述

Before plants colonized land from the sea, the continents were barren. Now millions of species of microbes, plants, and animals live on land, and it was the evolution of land plants that made all of this possible. 

To understand this staggering transformation, it is first necessary to understand what exactly plants are, and how they evolved to meet the challenges of life on land.

There are certain characteristics that all plants share, from the tallest trees to the smallest moss. First, all plants are multicellular eukaryotes. 

Second, plants produce the photosynthetic pigment chlorophyll in organelles called chloroplasts, enabling them to produce their own food using energy from the sun. Third, all plants have cells that are surrounded by walls made of cellulose. 

Lastly, all plants have a life cycle characterized by the alternation of generations, defined as the transition between haploid and diploid multicellular stages over the life cycle. Here, generations refers to two different multicellular phases in the life cycle. 

One phase is the haploid gametophyte. The gametophyte produces gametes by mitosis, which fuse during fertilization to form a diploid cell, which then undergoes mitosis to develop into a sporophyte. The sporophyte, in turn, produces haploid spores by meiosis, completing the cycle by giving rise to new gametophytes.

Although all plants share these characteristics, there are stark differences among the diverse plant lineages in how some of these characteristics are expressed. 

Land plants fall into three major groups, the nonvascular plants, the seedless vascular plants, and the seed plants. Each of these groups contains many thousands of species.

As evolutionary time progressed, the three groups evolved different traits which characterize them - from vascular tissue, to the evolution of fruits. Together, these adaptations enabled plants to dominate most terrestrial biomes.