STATE HORTICULTURAL SOCIETY. 223 



of the storm center and usually at a distance of from 250 to 800 miles therefrom" 

 As storm centres rarely pass more than 300 to 400 miles north of Central Minnesota 

 in the summer, the season of their occurrence, it is probable that a true, well-defined 

 tornado has never occurred north of the line of the Northern Pacific Railwa}'. 



The influence of climate on plant growth, whether for good or evil, depends on 

 the relative proportion of the different agencies which give to each part of the globe 

 its distinctive climatic characteristics. One of the most important, as we all tnow, 

 of these agencies in limiting plant growth, is the amount of moisture which the air 

 contains, the amount which is yearly condensed in the shape of rain or snow, and 

 the activity of evaporation, which again depends on the dryuuos i\.na temperature 

 of the air. The warmer air is the greater inherent capacity it has for containing 

 water in the shape of a vapor or gas, and therefore, its greater evaporating power; 

 and this being so, it would follow that the higher tlie annual average temperature 

 of a place the greater would be the need of an increased rainfall to supply the defici- 

 ency caused by an active evaporation, and consequently lost and not available fo 

 pl:i,nt needs. Minnesota has twenty-eight inches of precipitation in the form of 

 rain or snow during the year and Soutiiern Texas the same amount, yet Minnesota 

 has verdant pastures, fields'of waving grain, dense forests, full-voiced rivers, and all 

 those indications of a climate having a proper balance between the elements of heat 

 and moisture, while the Central Rio Grande Valley, being much warmer, is 

 posessed of essentially arid characteristics. A tcaveler commenting on this excess 

 of evaporation, saj'-s: "To-day we had a violent thunderstorm during which torrents 

 of water fell." Three days after he says: "From the effects of an imclouded sky 

 and a burning sun all traces of the heavy rain have disappeared ; the scanty vegeta- 

 tion is again drooping with drought and tlie earth a bed of dust." 



One inch of rain during the bland summer of the Red River Valley is ample for 

 plant needs for a fortnight. Provided the summers are warm enough to ripen the 

 wood, forestsin temperate regions seem to prefer the colder part of the temperate zone 

 where though the annual rainfall is small, as in the British Northwest, and it be 

 only from seventeen to twenty-two inches in the course of the j'ear, yet there are 

 found extensive forests of coniferous trees. That amount would be totally inadequate 

 for plant needs in the warmer regions, but in that countrj' the low annual mean tem- 

 jperature, by diminishing excessive evaporation, makes nearly all the precipitation 

 available for promoting forest growth. It is also evident that forests are best suit- 

 ed to those places where the rainfall is not restricted to a part of the year, leaving 

 the balance dry and endangering the integrity of the forests from fires during the 

 dry season. California and the North Pacific coast with their magnificent forests 

 of conifers would seem to be an exception to this but it will be remembered that 

 although California is almost absolutely without rain during the hot months, yet 

 the forests are there invariably found where from proximity to -the ocean or from 

 great altitudes, the absolute or relative amount of moisture in the air is so very 

 great that by diminishing excessive evaporation it practicallj' takes the place of rain, 

 and husbands the moisture derived from the heavy Snows and rains of winter 

 ihrough the long months of drought until the rains come again. The forest regions 

 of the North Pacific coast do indeed have a rainfall during all the months of the 

 year, but this is so small during the summer and so near the limit of excessive dry- 

 ness that a very small diminuition from the normal amount invariably produces 



