METEOROLOGY IN ITS RELATION TO HORTICULTURE. 109 



De Candolle further modified Boussingault's method in an important 

 particular by omitting to consider temperatures below the freezing-point. 

 He urged that since vegetation became dormant at those low temperatures 

 they could have no meaning as regards the growth of a plant unless 

 indeed they became so low as to destroy the plant altogether, and there- 

 fore he omitted from his calculations all temperatures below 32° Fahr. 

 Further than this, he showed that for each species of plant there is 

 a definite temperature at which its growth begins, and below which it 

 becomes dormant ; and therefore the first step towards finding the 

 thermal constants of any plant is to settle what is that lower limit of 

 temperature which marks the boundary betw^een activity and dormancy. 



This is not a very easy matter to determine with precision. It is 

 certainly not the same for all plants, but for temperate latitudes it seems 

 to be pretty generally conceded that the temperature of 42° Fahr. is a 

 sufficiently close approximation to the dividing line for general use— at 

 any rate for cereals and for most other farm crops. 



De Candolle held that the chief factor which in our hemisphere 

 governs the geographical extension northwards of any plant is whether 

 or not it can obtain a certain fixed amount of heat, proper to itself, 

 between the time when the mean temperature in spring reaches 42° and 

 the time w^hen in the autumn it again falls to that point. A good many 

 physicists, especially on the Continent, have tried to determine the 

 aggregate amount of heat required for the growth and maturation of 

 various plants. I cannot now summarise in any detail the different 

 methods followed and results obtained ; but as regards methods it must 

 suffice to say that, speaking generally, they have all proceeded more or less 

 on the lines of Boussingault and of De Candolle. 



In this country Dr. Gilbert, F.R.S., tried to determine it for wheat, 

 first by combining the temperature data observed at Greenwich Observa- 

 tory with the dates of harvest at Rothamsted, and also for later years by 

 applying to the Rothamsted data the temperature values published by the 

 Meteorological Office in its Weekly Weather Beport where they are 

 given in a form suggested by Sir Richard Strachey, F.R.S., as being specially 

 adapted to the requirements of agriculture, and to which he has given the 

 name of " day-degrees." The term " day-degree," following the analogy of 

 the "foot-pound," signifies one degree of temperature continued throughout 

 one entire day, and it will be best explained by the aid of a diagram 



(Fig. 21). 



44^ 



42" 



Mdi 



40' 



Fig. 21. 



If we imagine the extremes of this figure to represent a day from mid- 

 night to midnight, and the centre horizontal line the fixed base temperature 



