224 



buried in frozen soil and their crowns exposed to drying 

 winds, and it is in fact mainly responsible for the absence of 

 trees in the arctic zone and at high elevations. The temper- 

 ature of the soil depends partly on its physical properties, 

 sand possessing a far greater capacity for becoming heated, 

 for instance, than clay ; partly on its depth, the deeper layers 

 only being affected by prolonged winter cold or summer heat ; 

 partly on the aspect, and partly also on the presence or absence 

 of vegetation and the character of such vegetation. 



Temperature is the principal factor determining the dis- 

 tribution of the valuable Hill Forests in India containing 

 the Deodar and other important Conifers and Oaks. It 

 also appears to be largely responsible for the distribution 

 of Teak, no good natural forests of this species occurring north 

 of cold- weather (i.e., November-February) isotherm 65. An 

 important Indian tree which appears to require high temper- 

 atures throughout the year is the Red Sanders (Pterocarpus 

 santalinus). 



(4) LIGHT. 201. It has been noted in Part V 



above, see page 214, that light may injure plants by being (a) 

 too intense or (6) too feeble. On high mountains the light is 

 more intense than in the lowlands and is of importance in 

 regulating the distribution of plants at high elevations- On 

 the other hand the too feeble light makes it impossible for 

 many plants, e. g. grasses, to live in the undergrowth of a 

 dense forest, while a heavy growth of grasses may in turn, by 

 preventing the access of light, be responsible for the death of 

 tree seedlings. Many of our most important Indian trees 

 require a great deal of light and cannot grow well in the shade 

 of other plants, such as Teak, Sissoo (Dalbergia Sissoo), Khair 

 (Acacia Catechu), Babul (A. arabica), Blue Pine (Pinus excelsa), 

 and the Chir (P. longifolia), while others can stand a consider- 

 able amount of shade, e. g. Xylia dolabriformis, Sal and Sain. 



(5) AIR. 202. The humidity of the air is of 

 Humidity of great importance for plants, inasmuch as (1) the supply of 

 the Atmos- wa ^ er m the so {\ obtained from rain, dew, snow, hail and from 



DilGrC' 



the condensation of aqueous vapour depends upon it, and (2) 



it regulates the degree of transpiration from the aerial parts of 



plants (transpiration ceasing in saturated air and increasing in 



dry air) and of evaporation from the soil. Radiation of heat is 



also slower in moist than in dry air. As noted in' Part V above, 



Action of see page 215, winds may injure plants directly by uprooting, 



Winds. breaking and rending them, and also indirectly by increasing 



transpiration, The second mode of action is particularly 



