8o OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



the first to prove that the formation of nitrates in soil is due to the activity 

 of micro-organisms, since nitrogen-containing soil, in which this process 

 can take place, loses the power of inducing it if heated up to iio C, 

 but regains that power when non-sterilized soil is added to it, and, further- 

 more, since chloroform instantly arrests the process in question. 



Winogradsky first isolated these organisms. They flourish in a well- 

 ventilated, moderately moist, alkaline soil that contains nitrogen, at 

 temperatures between io C. and 45 C. According to Miintz, the nitrate- 

 bacteria play a prominent part in disintegrating rock, by penetrating 

 the finest pores and exercising their chemical activity .^ 



It may be regarded as established that bacteria in the soil enrich 

 this in nitrogen by utilizing free nitrogen from the air. Leguminosae 

 with root-tubercles containing bacteria, Elaeagnaceae, and Alnus also 

 have this power. According to P. E. Miiller's ^ investigations the moun- 

 tain-pine (Pinus montana), which has both mycorhizae and peculiar, 

 coralloid, branched root-tubercles, also belongs to the plants capable 

 of fixing free nitrogen. Experience has shown that where Picea excelsa 

 has been planted on heaths in Jutland it flourishes better in company 

 with Pinus montana than without it. It is probable that in this case 

 the mountain-pine provides the spruce with nitrogen. 



Bacteria do not flourish in a soil containing free acids (humous acids) ; 

 consequently they are scanty, or lacking, in peat and similar soils. 



CHAPTER XXI. EXPOSURE. OROGRAPHIC AND OTHER 



FACTORS 



The different factors already considered are in nature so varied and 

 connected bj^ such a number of transitions that the greatest diversity 

 results in the nature of habitats and in the differentiation of vegetation, 

 and it becomes extremely difficult to decide which factors are the most 

 weighty in a given case. But this multiplicity and variety are further 

 increased by modifications occasioned by certain geographical or 

 orographic factors. Among these are included the direction of mountain- 

 chains and valleys, the height of mountain-chains, the steepness and 

 exposure of declivities, and so forth. 



The direction and height of mountain-chains. These are of paramount 

 climatic significance. They steer wind into definite directions, occasion 

 fohn-winds, capture moisture from the wind on certain sides, and con- 

 dense aqueous vapour in higher regions in the form of clouds and rain ; 

 consequently, on certain sides or at a certain altitude above sea-level 

 luxuriant forests may prevail, whilst on other sides or at a lower level 

 extreme aridity reigns. Thus the coast-mountains of Brazil are rainy 

 and clad with forest ; but the interior is dry because the moisture of 

 the trade-wind is condensed and deposited before the interior is reached. 

 In like manner the coast of South Africa is moist, but the Karroos are 

 dry. In the West Indies the more low-lying islands are dry and receive 



^ See C. Schroter, 1904-8, p. 558. * P. E. Miiller, 1903. 



