662 DISTKIBDTION AS AFFECTED BY MOISTURE AND SOIL. 



great measure, the nature of the plants which grow at different heights. 

 The same changes take place as have been shown to occur in proceed- 

 ing 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 : — 



2. — Effects of Moistuee. 



The absolute and relative 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 exhibit 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 resist- 

 ing the effects of evaporation and transpiration. Among these may be 

 noticed Cactaceae, Mesembryacese, 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 maybe said to correspond with our winters. In some 

 parts of South America, where no rain faUs 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. 



The physical localities in which plants grow vary considerably. 

 These variations are connected with the dryness and moisture of the 

 son, as weU as with its mechanical and chemical composition. Some 

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

 peaty soU, others in sandy soil. Thurmann has endeavoured to show 

 that the nature of the soil, whether siliceous, clayey, calcareous, or 

 saline, has an effect in modifying the vegetation. Prof. E. Forbes 

 states that in Lycia he could easily distinguish the serpentine from 



