INFLUENCE OF HEAT ON CONFIGURATION AND DISTRIBUTION OF PLANTS. 527 



three years, at Innsbruck in the Tyrol, and in the eight points of the compass round 

 an isolated conical sand-hill, have shown the following mean temperatures: 



North. North-east. East. South-east. South. South-west. West North-west, 

 15-3. 17-0. 187. 20-0. 19-3. 18'3. 185. 15'0. 



The difference between the south-east and north-west amounts, according to this, to 

 not less than 5, and it is probable that at higher altitudes it would show even a 

 more marked increase. And herewith is connected the rising and falling of the 

 upper limit of vegetation on the different sides of a mountain. On slopes long 

 exposed to the sun the plants advance much further upwards than on the shaded 

 sides of a mountain, or those which are warmed by the sun's rays during only a 

 short time; and the difference of the upper limit on the north and south sides 

 oscillates in high mountain regions between 200 and 300 metres. It frequently 

 happens that species reach their upper limit on the north side at 2000 metres, 

 while on the south side not until 2400 metres is reached. From this it strikes us 

 that the contrast between the upper limit of plants on the north and south sides 

 becomes greater the higher we climb up into the mountain. In this respect a com- 

 parison of beeches and firs is very interesting. Beech trees (Fagus sylvatica) find 

 their upper limit in the Limestone Alps of the North Tyrol on an average at an 

 altitude of 1430 metres; on the sunny side of the mountains the beech limit rises 

 to 149 metres above this average, while on the shady side it falls short of the 

 average by 112 metres; thus the difference between the sunny and shady side for 

 beeches amounts to 261 metres. Norway spruces (Abies excelsa) find their upper 

 limit in the same region, on an average, at an altitude of 1777 metres; on the sunny 

 side of the mountain the spruce limit rises to 185 metres over, while on the shady 

 side it remains 125 metres below the average, and thus the difference between the 

 sunny and shady side amounts for spruce to 310 metres. Thus whilst in the zone 

 stretching from 1300-1600 metres, the difference between the shady and sunny 

 sides amounts only to 261 metres, it rises to 310 metres in a zone from 1600- 

 1900 metres, which again can only be accounted for by the rising intensity of the 

 sun's rays with the increasing altitude. 



From all this it may be seen how vegetation adapts itself to the given heat 

 conditions; how the smallest advantage offered in any spot is made use of; and 

 how much the form of the plant depends upon the conditions of warmth in the 

 habitat. 



The above statements also demonstrate that the distribution of plants on the 

 earth stands in the closest connection with the distribution of heat. In another 

 volume of this work an opportunity will be taken of discussing this connection 

 fully; here it is sufficient to mention that from the local conditions of warmth, viz., 

 from the elevation of the temperature of the soil effected in circumscribed spots in 

 mountainous districts by the sun's rays, the preservation of colonies of plants, from 

 earlier, warmer periods is explained. The largest part of the central European 

 uplands, especially the Northern Limestone Alps, exhibit colonies of plant-species 

 on limited areas, which are entirely absent in the immediate neighbourhood, and 



