220 THE PLANT WORLD. 



will soon be found that such increase lessons the rate and amount 

 of growth, and a continued increase will soon bring the thermom- 

 eter to a point where a supra-optimum will be reached at which 

 growth ceases. This may simply bring the plant to rest as might 

 the cold of autumn, and with but slight damage. Biit if the 

 heat be increased still further a third point will l)e found at 

 which the plant is killed and by such a test we will have ascer- 

 tained the point of fatal lieat. 



Starting again with a })lant at the optimum, it will l)e found 

 that as the temperature decreases, growth slews down until an 

 infra-optimum is reached at which growth ceases as it did at a 

 certain point above ; still, we believe this is the temperature of 

 fatal cold at which living matter is totally disorganized. 



Our efforts at acclimatization and our work in securing the 

 feature of hardness in plants, with respect to temperature, con- 

 sist in operations by which the position of the cardinal points of 

 the plant with which we may be working may be altered on the 

 scale of the thermometer. These cardinal points undergo wide 

 changes in a state of nature, and it is by taking the inherited 

 capacity for adaptation of any plant with regard to this partic- 

 ular into account that we may hope to make our greatest pro- 

 gress. First of all it is obvious that these five critical points in 

 the life of any plant change with the development of the indi- 

 vidual, and that the optimum slides up o.\ down to scale, or all 

 open out more widely. Take any plant such as the radish, 

 wheat, squash or sunflower, and it has been found that seed or 

 grains air-dry, and in resting condition, may endure the lowest 

 cold that can be produced, that of liquid hydrogen at about 454 

 degrees F. below the freezing point of water, which proves that 

 the fatal cold in such cases, is extremely low, and to have 

 only a theoretical existence. The same seeds in a resting and 

 dried condition may be subjected to the heat of boiling water at 

 212 degrees F., so that the points of fatal heat and cold lie far 

 apart in this stage of the existence of the plant, l^ow give them 

 a supply of moisture and start germination, and a radical change 



