ITS FLORA. 29 



of either pole.* The mountain that has its base waving 

 with the palms and tree-ferns of India, may have its sides 

 clothed with the oaks and pines of Europe, its higher cliffs 

 with the dwarf- willows and mosses of Nova Zembla, while 

 its snowy peaks are as void of life as the ice-bound shores 

 of the arctic circle. Besides these conditions, there are 

 others of site, or locality, or habitat conditions which re- 

 quire that the weeds of the ocean should differ from the 

 plants of the marsh, the plants of the marsh from the 

 herbage of the open plain, and the verdure. of the plain 

 distinct from that of the mountain forest. Nay more : 

 there are some tribes that will nourish only in rich organic 

 mould, others that prefer the shingly surface of the arid 

 desert ; some that exist only on calcareous soils, and others 

 unknown beyond the limits of the salt marsh. Wherever 

 the prime conditions of heat, light, and moisture are present, 

 there the vegetable germ manifests itself here incrusting 

 the naked rock, there mantling the surface of the stagnant 

 pool now rooting itself in the decay of its own kind, and 

 at times finding a habitat even in the tissues of the animal 

 structure. More than this : every climatic influence, how- 

 ever faint, leaves its impress on vegetable life. A thicker 

 layer is added to the concentric growth of the timber-tree 

 during a genial than during an ungenial summer ; the 

 southern slopes of a hill are more verdant and flowery than 

 those of its northern side ; some plants luxuriate in the 

 sea-breeze which would be death to others ; and the leafiest 

 side of a tree is ever that which is most accessible to the 

 open sunshine. Again : plants that grow in localities 

 marked by sudden extremes of heat and cold are always 

 more variable in stature, habit, and foliage, than those 

 which flourish under the steadier influences of a genial 



* The capacity of the atmosphere for heat decreases with its density, 

 and this density decreases from the level of the ocean upwards. 



