These inter-relations of outward conditions and internal adaptiveness bring their advantages and their disadvantages. It is an advantage to annuals that they require only a short part of the year for their full development, and their existence is not endangered by winter frosts or the heats of tropical dry seasons; also that the abundance of seed, which the devotion of their whole life to one effort makes them capable of producing, enables them to spread very rapidly, and occupy all favorable situations in a few generations. It is, on the other hand, a disadvantage that they have to conquer their ground every year, and the tenderness of the structure of their parts above-ground renders them liable to destruction by animals while they have not vigor enough to send forth new shoots. Their continuance is also sometimes endangered by the occurrence of an exceptional season too short for them to mature their fruit.
The least advantageous habit is that of those plants that grow for several years, and produce fruit but once, and it is of the rarest occurrence. Exceedingly well off are tuberous and bulbous plants, whose under-ground perennial organs are protected from the influence of climate and the attacks of animals, have stored in them quantities of reserve food, and are fitted to send out new shoots as soon as favorable weather sets in, and to mature their fruit in a very short season. Hence we find such plants common where the seasons of growth, whether on account of the frigidity or the dryness of the climate, are shortest. They also seem to prevail where the climate is moderately moist and warm; and nearly all water phanerogams are of this class.
It is to the advantage of woody plants that they do not have to build up every year so much of the scaffold on which their flowers and fruits are to grow. If they have evergreen leaves, these contain, at the end of the season, a quantity of reserve food fit for assimilation, and are ready to go to work absorbing more immediately on the opening of a new season. They have also the title of possession in the struggle with other plants, and are protected against animals. They are, however, more exposed to injury from changes of climate than other plants.
Thus, each life-habit has its advantages and its disadvantages; and we may conclude from this that a change from a long-lived to a short lived form, or the reverse, may take place in consequence of adaptation to changes in outer circumstances. The origin of new habits depends, however, not on external circumstances alone, but upon a certain disposition in the plant, by means of which it is able to adapt itself to new conditions.
The climate has the most important influence upon vegetation, especially upon its endurance and its life-habits; for temperature, moisture, and light are the conditions on which the life of the plant depends in all its stages; and the movements of the air have also their effect upon its existence. When the climatic conditions change, the plant must
