ESTIMATION OF THE UEAT NECESSARY TO GROWTH. 



561 



From tliese observations it is easily gathered that in those instances in which 

 the seed was exposed to a higher temperature, only a portion of the heat supplied 

 was actually employed in germination, and that, therefore, the constants calculated 

 on the basis of these observations are much too hirrh. 



If the thermometer could tell us the amount of heat actually needed within a 

 certain time b}' plants growing togethei", then only might the constants reckoned 

 from these readings be regarded as accurate and become useful for comparison. But 

 these conditions are not fulfilled. Usually here the conclusions are only post hoc 

 propter hoc. Thermometric readings are bi-ought into calculation which include the 

 surplus of heat not used by the plants, and consequently the constants are not the 

 correct expression of the quantity of heat actually consumed in growth. 



The bases on -which the calculations are founded, for growing organs directly 

 under the influence of the sun's rays, are much more uncertain than those for seeds 

 germinating in shaded ground. Besides, doubts must arise from the fact that the 

 sun's rays have a widely differing effect on foliage, flowers, and fruits from that 

 which they have on the mercury of the thermometer. This defect may indeed be 

 removed by using the same instrument in all observations and employing suitable 

 corrections; but it is a more serious matter that we have no means of ascertainine- 

 how much light is changed into heat in growing organs exposed to the sun's rays. 

 With increasing altitudes the intensity of the light increases, and its significance for 

 growth increases in a corresponding manner. But it is impossible to determine 

 these relations numerically, more especially to determine them in plants and 

 thermometers observed in the open. 



Nor must it be forgotten that the absorption of heat also depends upon the 

 individuality of the plant observed, and upon the constitution of the protoplasm of 

 the particular species. The seeds of the White Mustard are incited to growth even, 

 by temperatures little removed from the freezing-point, while the seeds of melons do> 

 not germinate until they have been exposed to the influence of a temperature of 

 18-5° C. for at least 17 days. This shows that every species has to a certain extent 

 its own zero at which growth begins, and all calculations of the heat required for 

 the growth of the stem and foliage of any particular species should always be 

 reckoned only from this zero. Moreover, it is a matter of experience confirmed by 

 all gardeners that higher temperatures are required for the development of flowers 

 than for foliage, and that for the proper ripening of seeds higher temperatures still 



Vol. L 35 



