PERIODIC PHENOMENA OF VEGETABLE LIFE. 89 



Alpine valleys, especially towards the limits of cultivation, the 

 effects of temperature are much less disturbed. In cold summers, 

 in which the depressions of temperature are often very consider- 

 able, the higher points of the limits of cultivation are brought 

 down to a much lower level for the year; even if the corn does 

 ripen, most of the chaff remains empty and the grains are very 

 small and unproductive ; a phenomenon always observed when 

 attempts at cultivation at great elevations have failed. As an 

 increase of temperature brings on an acceleration of the develop- 

 ment of vegetation, so it is observed, that fruitful years in the 

 Alps are generally those where the ripening of corn takes place at 

 an earlier period than usual. 



Variations in produce, depending on circumstances of weather, 

 are, as we have already remarked, very great at the limits of 

 Cereals. Between these higher points and the plains, even 

 between the bottoms of different valleys, very perceptible differ- 

 ences are observable in the relative fertility of different years. In 

 the former case, for instance, the want of summer rains is less 

 felt than in the plains, as in mountains the place of rain is 

 partially supplied by dews and by the moisture of the soil, whilst 

 at the same time the high temperature, which generally accom- 

 panies dry summers, promotes considerably the ripening of the 

 grain. This remark relates, however, chiefly to elevated points 

 in the northern and central Alps, whilst, in the warmer masses 

 on the southern declivity, dry summers may diminish much the 

 produce of com among the mountains. 



Recapitulation. 



1. The duration of the period of vegetation diminishes with 

 altitude ; it is only ninety-five days at an elevation of from 7000 

 to 8000 feet, and at the outer limits of phsenogamous plants 

 above 10,000 feet, it is reduced to about a month. These highest 

 plants, in unfavourable years, remain covered with snow the 

 whole summer. 



2. The time which intervenes between sowing and harvesting 

 of winter corn increases with altitude, and at the outer limits of 

 Cereals, at an elevation of 5000 to 5200, often extends to a whole 

 year. 



3. The retardation of vegetable development is generally less 

 in the flowering than in the ripening of fruit ; it amounts in the 

 Alps for every 1000 feet in elevation to ten days for the former 

 period, twelve and a half days for the latter; and for the whole 



