GEOGRAPHICAL DISTRIBUTION OF PLANTS. 41 



142. The temperature of the natural stations of plants is always such as 

 to enable the species to continue itself by seeds ; but as, in a state of culture, 

 plants can be propagated by various modes which do not require the pro- 

 duction of either flowers or fruits, it follows that in any given natural 

 station a great many plants may be cultivated by art, which could not 

 exist there in a state of nature ; and which, if introduced by art, and not 

 continued by the same power, would perish with the life of the individual. 

 Hence the immense number of species, from all parts of the globe, which 

 will grow in the open air in Great Britain, and which, if the island were to 

 relapse into a state of barbarism, would for the most part disappear. Hence, 

 also, by the artificial climates of our plant structures, we can grow and 

 propagate all the plants of the world, though there are many that for want 

 of space cannot attain their natural magnitude in such structures. The 

 mere fact, however, of our being able to grow tropical plants in air arti- 

 ficially heated, shows that temperature has a greater influence on vegetation 

 than any other element of growth. 



143. The influence of light on the distribution of plants is very consider- 

 able. As heat and moisture are the chief agents in calling the vegetable 

 germ into existence, so, the plant once developed, light is the grand sti- 

 mulator of vitality ; causing, by its influence on buds and leaves, the ab- 

 sorption of the sap by the roots, and the exhalation of water and decompo- 

 sition of carbonic acid by the leaves. It is probable, as Professor Henslow 

 conjectures, that each species requires a different degree of light as well as 

 of heat ; and though no general laws have yet been discovered on this sub- 

 ject, we find that succulent, resinous, or oily plants, and all plants with 

 needle leaves, prefer situations where they can obtain much light ; while 

 almost all evergreens, except such as are needle-leaved, prefer situations 

 somewhat shaded. As the density of air is diminished as we ascend in 

 the atmosphere, so the intensity of light is increased ; and it has been sup- 

 posed that as high elevations correspond with high latitudes in regard to 

 heat, they ought to correspond also in regard to light ; though this has not, 

 as far as we know, been determined by facts. But it is clear, from what 

 has been stated, that in any given latitude the plants which grow on plains 

 receive less light than those on mountains ; and that the two extremes, in 

 any country, are the sea- shore and the line of perpetual snow. The mean 

 distribution of light is unquestionably much more equable in all latitudes 

 than the mean distribution of temperature ; but the extremes, in its mode 

 of distribution, are remarkably different. Plants in the northern regions 

 generally are covered with snow more than half the year ; and those which 

 reach above the snow, such as the trees, have perpetual sunshine for 

 several weeks together during summer, and the absence of the sun for a 

 similar period during winter. In all countries where snow falls, and rests 

 on the country for some weeks or months, the mean degree of light received 

 by herbaceous plants, such as the pasture grasses, must be considerably dif- 

 ferent from the mean light received by the same species hi climates where 

 snow is unknown ; but as in all cases in which light is so entirely excluded 

 from plants in a natural state, vegetation is dormant, or nearly so, plants 

 escape uninjured. From these facts, some valuable deductions may be 

 drawn as to the light which plants require, or may dispense with, in a state 

 of culture. 



144. The influence of water, whether in the soil or in the atmosphere, on 



