HOW PLANTS LIVE
PLANT life was the first life on earth. The first primitive plants appeared in the seas over 600 million years ago, but more than 160 million years went by before plants appeared on land. These plants were simple, without roots, and as yet leafless, but their presence on land made possible the emergence of animal life from the sea, by providing a source’ of food. All the brilliant and complex variety of the plant kingdom has evolved from these first land plants.
A plant is generally considered to be a green, living thing with roots in the soil and with stem, leaves and flowers above ground. Yet a plant is not necessarily green, and it may have no roots, leaves or flowers. Flowering plants comprise only one of the thirteen divisions into which botanists now classify the plant or vegetable kingdom. The twelve other divisions consist of more lowly and less evolved plants, such as bacteria, algae, diatoms, fungi, mosses, liverworts, horsetails and ferns, all known collectively as cryptogams or non-flowering plants.
All plants are alike in that they are made up of living matter called cells, are capable of living on inorganic sub-stances, have no power of locomotion to give them freedom of movement, and no specialized sense or digestive organs.
The simplest plants consist of a single cell able to live, breathe, feed, grow and reproduce by itself. Many bacteria and algae are in this group. But in the course of evolution plants have become increasingly complex, the most highly evolved and complex being the flowering or seed-producing plants — the familiar plants ofand countryside. These plants have a well-defined structure of root, stem, shoot, leaf, flower and seed, though the form of their structure is variable and diverse.
There are more than 250,000 species of flowering plants in the world, and these are split into two divisions — the first, plants with naked or incompletely protected seed (gymnosperms), which include the coniferous trees such as pines, firs and cypresses, and some plants, now extinct, which are only known as fossils; and the second, plants with seeds enclosed or covered in their ovaries (angiosperms), which include the vast majority of all garden and farm plants. Angiosperms are themselves divided into two groups. The first is the monocotyledons — plants with seeds containing one embryonic seed leaf or cotyledon; with many tufted roots of equal length and thickness; with leaf veins parallel and equal in size and thickness, and flower parts in threes or multiples of three. The second group is the dicotyle-dons — plants with seeds containing two embryonic seed leaves; with a root sys-tem of a main central taproot and side roots branching from it; with leaves containing a main vein and lateral veins branching from it and a network of fine veins between, and flower parts in fours or fives, or their multiples.
Seeds of different plants vary greatly in size, shape, texture and colour, but all have the same basic structure. Every fertile seed is a dormant embryonic plant, and is hard and dry and covered with a protective coat.
Within the seed coat lies a food storage organ, called the endosperm, and the embryonic plant itself with its primary leaf or leaves (the cotyledons). In many seeds these primary leaves fill much of the space and contain most of the food reserves for the embryonic plant. They are the two halves found in seeds like, and walnuts. Between and attached to them is the embryonic plant, sometimes called the germ, from which root and stem develop when the seed germinates.
To germinate or begin to grow, a seed needs air, moisture, darkness and warmth. The degree of warmth required depends on the origin of the plant. Seeds of tropical or subtropical plants need higher temperatures than those of plants native to the temperate zones.
Moisture and air enter the seed through a minute hole (the micropyle) in the seed-coat near the tiny scar or hilum that shows where the seed was attached to its parent plant. The seed softens and swells, the seed-coat splits, and the first growth to emerge is the young root or radicle. This grows rapidly. If it is a dicotyledonous plant other roots are formed branching from this original one, but if it is a mono-cotyledonous plant the root is quickly joined by other roots growing from about the same point of origin.