558 



ESTIMATION OF THE HEAT NECESSARY TO GROWTH. 



but that those alterations which really constitute growth are not produced in 

 the cells of the embryo at this temperature. Not until the temperature of the 

 soil rises above 15° does the embryo elongate, and the radicle bore its way through 

 the seed-coats. But all these figures would give by themselves a very incomplete 

 idea of the heat-requirements of germinating seeds, were it not also ascertained 

 how long the seed must be exposed to the given temperatures in order that its 

 embryo should increase and develop. If a hen's egg is exposed for only two or 

 three days to a temperature of 35° to 40°, it will not be hatched; hatching can 

 only take place if the egg remains for 20-21 days under the influence of this 

 constant temperature. With seeds the case is the same. The following is a 

 selection of the results obtained in this relation: — 



If the number of days is multiplied by the temperature, the product may be 

 looked upon as an empirical formula for the heat necessary to the process of 

 germination. It may be considered that this product is of regular amount, and 

 it is regarded as a "thermal constant". Thus, for purposes of comparison, the 

 thermal constants for the germination of the seeds of the Gold of Pleasure might 

 be expressed as 18'4, of the Poppy 46-0, of Maize 115'5, and so forth. 



In these calculations, of course, only the constant temperatures of the soil 

 not directly illumined by the sun's rays come under consideration. The matter 

 becomes far more complicated when it is a question of determining the constants 

 for other stages in the development of plants, such as the bursting of foliage 

 from the buds, the opening of the first flowers, and the ripening of the first fruits. 

 These phenomena of growth in the majority of plants in the open do not occur 

 in the shade, but in the sun. Moreover, in the places under observation, the 

 temperature is not constant, but changes from hour to hour, attaining its minimum 

 shortly before sunrise, and its maximum in the first hours of the afternoon. Since 

 experience has shown that the extent of growth is regulated according to the 

 highest temperature in the sunshine, it follows that neither the shade temperature 

 nor the mean temperature, but the readings of the maximum thermometer, exposed 

 to the sun, must be used for the estimation of the constants in the above-mentioned 

 phenomena of growth. 



To obtain the thermal constants for foliage-production, flower-opening, and 

 seed-ripening, of a plant growing in a situation illuminated by the sun, one must 



