THE PLANTS

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most paleobotanists believe that land plants evolved about 430 million years ago from predominantly freshwater green algae. Living members of these groups seem more evolved today, so it is assumed that some of their traits were developed after they transitioned to land. Primitive plants were simple structures that did not look like modern plants.

The earliest plants had upright stems but no roots and leaves, to say nothing of flowers, a development that would come much later. Nonflowering plants fall into two groups: bryophytes and vascular plants. Bryophytes lack a system for the transport of water and food. They

 tend to be small and lack true roots. They photosynthesize and mostly reproduce by means of alternating nonsexual and sexual generations, in fashion similar to the ferns. Early nonflowering vascular plants include the ferns and horsetails. These plants reproduce by means of spores

and alternating generations. Also included in this group are the gymno- sperms (“naked seeds”), plants whose seeds are not enclosed, as in flowering plants, but sit on the scales of cones. Conifers, including pines, firs, and spruces, create both male and female cones. The male cone makes fine pol- len that is blown onto a female cone and unites with an egg inside, producing a seed. When the seeds ripen, the scales loosen and spread out, allowing the seeds to disperse. Some 200 million years ago, gigantic gymnosperms formed the dominant plant life on Earth. As such they also satisfied the appetites of the Jurassic herbivores.

A number of plants today appear little changed from their prehistoric predecessors: mosses, horsetails, and ferns. found especially in moist environments, for example. Others are found among the cycads, a group that goes back 245 million years. Cycads have a columnar trunk and a crown of leaves, much like a palm tree, and are either male or female. They are found in the tropics and sub- tropics of both the Eastern and West- ern Hemispheres. Both sexes of cycads in most species bear outsize cones. To- day’s gingko, a lone survivor of a once

large group, no longer appears in the wild. It formerly was limited to south- eastern China, although as a cultivated plant it is known worldwide. Gingkos shaped leaves. By about 380 million years ago, are recognized by their clusters of fan-plants were diversifying into the forms we know today. As specialized tissues to transport water and nourishment and provide strength in the stems developed, tree-size and treelike species were able to thrive. Seed like structures soon appeared, leading to the development of flowering plants.

Ferns reproduce differently from the flowering plants that bear seed. Under a fern frond, or leaf, are often found rows of small brown dots called sporangia. Inside these sporangia, spores develop and release into the air when they are ripe. Fallen spores sprout into tiny, of- ten heart-shaped plants that anchor themselves in the ground with root- like rhizoids. Under their leaves are separate structures where eggs and sperm develop and mature. Rain swells the sperm structures and they burst, releasing flagellated

: sperm that travel to the egg in water droplets. Fertilization of an egg by sperm results in a new fern plant with a core of fronds that usually start out tightly coiled and eventually unfurl. Fern species have existed on Earth for some 300 million years. They thrived on Earth from 359 to 299 mil- lion years ago during the Carboniferous period, which is sometimes called the age of ferns, since they were then the dominant vegetation. The ferns that grew during the Carboniferous period are now extinct,