662 THE POPULAR SCIENCE MONTHLY. 



disappear altogether. Thus the persistent, often-fruiting species gain 

 the monopoly on high mountains and in Arctic regions, but with the 

 difference that in some districts they maintain themselves above- 

 ground through the whole year without protection against the climate, 

 while in others they exist through a long period of rest, protected 

 against the injurious effects of the cold by means of their perennial 

 parts under the soil or under the cover of an effective shelter. 



Without attempting a closer review of these relations, or of the 

 development of different life-terms in the successive periods of the 

 development of the earth's surface and of the vegetable kingdom, we 

 come to these comprehensive conclusions : A plant has to pass through 

 two phases, one of vegetation, the other of propagation, both of which 

 are conditioned upon the supply of food. In the simplest instances 

 these phases are exhibited in one and the same cell, which, absorbing 

 the food, elaborates it for its own growth, and gathers so much force 

 that two new individuals are produced by division while the mother- 

 plant ceases, by this fact, to be. In another yet similar manner is 

 accomplished the life of those plants which in their complicated 

 structure present an extreme conti*ast to the simple cell. In this case 

 also the whole feeding of the plant is performed with the sole object 

 of adapting it to the purpose of propagation ; but the end is reached 

 in various ways and times, while the attainment of it is not always ac- 

 companied with the end of life. Some plants proceed in uninterrupted 

 growth from their origin to the end of the propagation, collect forces 

 on forces, and die exhausted as soon as propagation is accomplished. 

 Others can not reach maturity by a short stage, but require a longer 

 time for gathering their forces. This enforcement is gained either in 

 an uninterrupted course, or, as is more frequently the case, by a series 

 of alternate periods of growth and rest, followed by a period of vigor, 

 when maturity of the fruit is at last reached and the plants die from 

 exhaustion. A third group reach the stage of propagation after a 

 shorter or longer period without giving up their whole strength to the 

 formation of seed, but apply a portion of the nourishment they re- 

 ceive to the formation of perennial organs by the aid of which they 

 continue to grow and become fruitful again, and conrpetent to repeat 

 the same processes an indefinite number of times. 



Seeing different plants thus attain very different ages, we naturally 

 inquire what are the causes of the diversity. They depend partly on 

 the adaptation to external conditions induced by climate, soil, and en- 

 vironment, all of which, especially climate, determine the life-habits of 

 plants in various ways, and seem also to affect the length of their life. 

 But these external conditions can not of themselves force the plant to 

 adapt itself to them ; it must itself have a fitness to react upon influ- 

 ences acting upon it from without. This is really the case in various 

 degrees, for the variability which a species exhibits by its individuals 

 in different directions extends to the length and the manner of its life, 



