Eugene Yang, age 9, of Louisville, Ky., for his question:
WHEN WAS THE FIRST PLANT FORMED?
It would be impossible for us to live without plants. The oxygen in our air comes from plants. The food we eat either comes from plants or from animals that eat plants. Some plants cannot be seen without microscopes while others are the largest of all living things on earth. And you'll find plants growing in just about every part of the world.
More than 350,000 different kinds of plants can be found growing on earth. More than half are flowering plants which include most fruits, grains, herbs, shrubs, trees and vegetables. They produce flowers, seeds and fruit.
Cone bearing plants, also called conifers, belong to.a separate class of plants. Most have needlelike leaves.
Another large grouping of plants is the fern family.
Scientists have found fossils of plants which grew more than 3 billion years ago. Botanists believe there were probably algalike plants growing in the oceans during the Precambrian period more than 4 billion years ago. We don't know exactly, however, when the first plant appeared on earth.
Indications seem to point toward the first land plant appearing on earth about 435 million years ago during the early Paleozoic Era. By the late Paleozoic Era, around 345 million years ago, great forests of mosses and seed ferns covered many sections of the earth.
Conifer, cycad and ginkgo trees became plentiful during the Mesozoic Era which started 225 million years ago. At this time the dinosaurs were roaming about the earth.
At the start of the Cenozoic Era, about 65 million years ago, great forests of the ancestors of many modern species of plants appeared. By about 26 million years ago, flowering plants and trees resembling those of today had appeared in Africa and Asia.
Through the years, some species of plants have grown special features that help protect them from plant eating animals. Noteworthy in this class are the spines on cactus plants and the sharp thorns on hawthorn and honey locust trees. Among the other plants with built in protective devices are the stinging nettles, poison ivy and poison sumac shrubs.
Natural selection has changed some plants so they can survive in their environments. Cacti, for example, have developed a way for collecting and storing the small amount of water, that becomes available. A network of roots spreads out over a large area just below the surface of the ground. These roots can quickly absorb water from even the lightest rain or short flash flood. The fleshy stems of the cacti store the water.