According to their length of life plants are classified as, or perennials. An plant is one which completes its life cycle within a year, a takes two years to complete it and a perennial is one that lives for many years. An , such as mustard or cress, germinates, grows up, and sets during spring and summer, and then dies. Such plants pass the winter safely in the form of dry . The chickweed and groundsel have a very short existence, for they only live five or six weeks, and consequently several generations may occur in a season. Such plants are called ephemerals.
The carrot, beet and turnip are examples of biennials. In the spring and summer of the first year of aleafy shoots are developed and the food manufactured is passed down the to be stored. The food is stored in the swollen tap of the carrot, while in the beet and turnip the storage organ represents the and part of the . During the winter most of the of these plants die down and the buds at the apex of the storage organ are protected by the bases of the dead . In the following spring a long leafy shoot grows up from the storage organ, and sets seed. The food store is entirely used up in the production of the flowering shoot and , and at the end of the summer the whole plant dies. In some biennials, such as the foxglove and mullein, the leaves do not all die in the winter of the first season, but some survive and are arranged in the form of a rosette closely pressed to the ground. The low-lying gives the leaves a certain amount of protection from wind and frost.
Perennials are able to live for many years, because each year after flowering and seed formation they are able to store up sufficient food to provide for the growth of new shoots in the following spring. Although most perennials flower every year, trees often do not flower until they are ten to twenty years old.
Herbs, Shrubs and Trees
According to their size and structurecan be classified as , shrubs or trees. Shrubs and trees are distinguished from herbs by their larger size and woody
THE HIGHER LAND PLANTS structure, and also by the fact that their aerial woody parts persist throughout the winter, whereas those of herbs usually die down at the beginning of this season. A tree is a woody plant with a main stem or trunk which usually does not form large branches until it has reached a height of several feet at least from the ground. A shrub, on the other hand, has a stem which divides into branches at a level close to the ground, and is altogether smaller than a tree. However, no hard and fast distinction can be drawn between a tree and a shrub, for there are plants, like the hazel, which sometimes remain shrubs, but occasionally grow into small trees. Further, the size of a tree is limited by the climate and the depth and nature of the soil.
Annuals are herbaceous plants, and not only the stem but the whole plant dies at the onset of autumn and winter. Biennials also are herbaceous, but perennials may be herbs, shrubs or trees. The lupin, Michaelmas daisy and delphinium are examples of herbaceous perennials, and have weak aerial, several feet in height, which die down in the autumn. The wallflower is a perennial which lives for several years, but although it is herbaceous in appearance its stems are woody to some extent and do not die during the winter. Its leaves also, with the exception of those on the older parts of the stem, do not drop off but persist through the winter. In the case of many trees and shrubs, however, the leaves drop off during the autumn ; such plants are said to be deciduous.
Trees and shrubs which retain their leaves during the winter are termed evergreens.
Ash, oak, elm, beech, birch, sycamore, horse chestnut, willow, larch, plane.
Scots pine, silver fir, holly, cypress, cedar, yew.