660 THE POPULAR SCIENCE MONTHLY. 



These inter-relations of outward conditions and internal adaptive- 

 ness bring their advantages and their disadvantages. It is an advan- 

 tage 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 capa- 

 ble 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 some- 

 times endangered by the occurrence of an exce]3tional 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 occur- 

 rence. Exceedingly well off are tuberous and bulbous plants, whose 

 under-ground perennial organs are protected from the influence of cli- 

 mate and the attacks of animals, have stored in them quantities of re- 

 serve 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 open- 

 ing 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 dis- 

 position 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, espe- 

 cially 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 



