DISTRIBUTION AS AFFECTED BY MOISTURE AND SOIL. 563 



from the equator to the poles. The following observations made on 

 the growth of certain trees on the Grimsel, show the relation between 

 height and latitude: 



Lat. On the Grimsel. 



Beech, which extends to 60 grows about the height of 3000 feet. 



Oak, 61 2600 



Fruit Trees, 63) 



Hazel, 64; 340 ~ 



Norway Spruce, 67 40' 5000 



Scotch Fir 70 6000 



Birch 70 40' 6400 



2. EFFECTS OF MOISTURE. 



1 138. The quantity of moisture in the air has a decided effect on the 

 distribution of plants. Nothing checks vegetation more than extreme 

 dryness. Hence the barrenness of those hot sandy deserts, which ex- 

 hibit only an arid waste, without a single blade of grass to relieve the 

 eye of the weary traveller. In warm and dry climates, succulent 

 plants occur, with hard epidermal coverings, capable of resisting the 

 effects of evaporation and transpiration. Among these may be noticed 

 Cactacese, Mesembryaceae, Euphorbias, and some of the Aloe tribe. 

 In the districts of Australia, where a dry climate prevails, many plants, 

 such as Proteas, Banksias, and leafless Acacias, have hard and dry 

 foliage, capable of enduring much drought without injury. In warm 

 climates, the effect of the dry season on vegetation is very remarkable. 

 This season may be said to correspond with our winters. In some 

 parts of South America, where no rain falls for eight months of the 

 year, the leaves during the dry season fall, buds are developed in their 

 axils, and it is only when the wet season arrives that the trees become 

 clothed with verdure, and the herbage appears. Forests appear to 

 keep up the humidity of the atmosphere in a country, and thus have 

 a powerful influence on the climate. 



3. EFFECTS OF SOIL, LIGHT, AND OTHER AGENTS. 



1139. The physical localities in which plants grow vary consider- 

 ably. These variations are connected with the dryness and moisture of 

 the soil, as well as with its mechanical and chemical composition. Some 

 plants are fitted to grow in water, others in marshes ; some grow in 

 peaty soil, others in sandy soil. The nature of the soil, whether sili- 

 ceous, clayey, calcareous, or saline, has an effect in modifying the vege- 

 tation. Prof. E. Forbes states, that, in Syria, he could easily distinguish 

 the serpentine from the limestone, not merely by their geological char- 

 acters, but also by the disposition of the arborescent vegetation. On 

 the serpentine, usually pines only grew, and never in thick forest masses, 

 but scattered; whereas the limestone bore thick clustered oaks and a 

 luxuriant underwood, with now and then clamps of lofty pines. In 

 the low countries near the sea, the serpentine was marked by Senecio 



