520. THE VEGETABLE WORLD. 
the same for all plants. Meleze and Dwarf Birches resist cold of 
40° Cent. below zero. While many of the Palms, the Orchids, 
and Tree Ferns die when the thermometer descends to 10°. 
While Alpine or northern plants subjected to the same temperature 
of 10°, fade and shrivel after a few days’ exposure, other plants 
accommodate themselves to the burning sands of Africa, whose 
temperature may be from 60° to 72° Cent. 
Another point of some importance to consider is the thermo- 
metrical degree at which each species begins to vegetate; thus 
the charming Soldanella of the mountains germinates and prospers 
at zero, while the Cocoa-nut-tree and other vegetables of the torrid 
zone remain unmoyed until the temperature of 15° or 22° is 
attained. 
Again, when te has commenced, what is the tempera- 
ture necessary to develop the flowers and ripen the fruits ? 
The Barley plant (Hordeum), the cereal which extends farthest 
towards the north, begins to vegetate when the thermometer 
ranges at 5° C. If, then, we would determine with precision 
the amount of heat which a plant must accumulate in order to 
accomplish its various processes of germination—budding, blos- 
soming and fruiting, and maturing its fruit—it is not necessary 
to take account of the lower temperature, but adding the mean 
temperature of each day on which the thermometer has exceeded 
that degree, we find that in high latitudes the barley plant 
ripens when it has received an aggregate amount of heat equal to 
1,500°. In order to produce death in wheat, an accumulation of 
heat equal to 2,000° is required. The vine, in order to produce a — 
drinkable wine, requires the accumulated heat of 2,900° Cent., the — 
point of departure being a mean heat of 10°. 
We can now comprehend why certain vegetables live in some 
countries without flowering and others without bearing fruit. The 
short summers and short days in such countries fail to yield the 
aggregate amount of heat, and that supplied is just sufficient to— 
develop their leaves, but not enough to expand their flowers ; 
and their fruits are abortive. The influence of heat on vegetation 
is so marked that we can scarcely name a single species which is 
truly cosmopolitan. Most vegetables occupy a determinate zone ~ 
of their own, which they rarely is The cold prevents them 
