METEOROLOGY IN ITS RELATION TO HORTICULTURE. Ill 



after correcting their results for method so as to make them fairly com 

 parable with Gilbert's. 



This correction was made by Gilbert in three cases, with the result 

 that his value of 1,901° Fahr. compared with that of 2,8G9° Fahr. obtained 

 by Boussingault, 1,851° Fahr. obtained by Herve-Mangon, and 2,192° Fahr. 

 oijtained by Risler. These values were got from data obtained in different 

 countries and in somewhat different ways ; and although they show 

 differences, yet they are so nearly alike as to leave little room for doubt 

 that an approximately uniform amount of heat is required by wheat for 

 the purpose of its growth and maturation. 



It seems to me, however, that for this inquiry it is not right to deal 

 with means such as Gilbert and the Continental physicists used, or with 

 averages obtained from data derived from a large area of country which 

 may present considerable climatic differences, such as Gilbert used in the 

 later phase of his work. And, further, I think the calculation ought to 

 extend over the entire period between sowing and harvest, because there is 

 no doubt that a certain amount of growth proceeds under favourable con- 

 ditions which may, and in our climate do, present themselves intermittently 

 before the spring has fairly and definitely set in. 



I have therefore made some calculations in which I used the day- 

 degrees got from temperature observations actually made at Rothamsted, 

 combined with the dates of sowing and harvest there, which were very 

 kindly supplied to me by Mr. Hall, M.A., the Director of the Lawes Agri- 

 cultural Trust. 



The mean amount of accumulated temperature above 42° required for 

 the ripening of autumn- sown wheat which I obtained from the data for 

 twenty-eight years is 1,961 day-degrees Fahr., and in sixteen out of the 

 twenty-eight years dealt with there is no difference from this mean 

 exceeding 2 per cent., although the interval between sowing and reaping 

 has varied by as much as seven weeks, the average being 285 days. 



For the present year (1905-6) I have slightly varied my plan, and have 

 obtained the amount of temperature by integrating the continuous record 

 of a Richard's thermograph kindly lent to me for the purpose by Dr. Shaw, 

 F.R.S., the Director of the Meteorological Office, and which I have had in 

 use at Warlingham, in Surrey, for the period between sowing and harvest. 



Warlingham is over 600 feet above sea level, and the climatic conditions 

 are certainly very different from those of Rothamsted ; nevertheless the 

 total amount of heat for the entire period of 282 days comprised between the 

 sowing and harvesting of wheat is 1,927 day-degrees, which again differs 

 less than 2 per cent, from the average obtained from the Rothamsted data. 



I cannot now deal with the effects of direct sunlight upon vegetation, 

 which are certainly Tery great, nor with the effect which the temperature 

 of the soil has upon the roots which draw their nourishment from it. 

 The temperature of the soil is probably of secondary importance to that 

 of the air ; for it seems probable that the various parts of a plant are acted 

 upon by heat independently, and without reference to each other. For 

 example, portions of a plant which are led into a greenhouse will grow 

 when other portions of the same plant left uncovered will show no signs 

 of life, the roots common to both portions being left in the soil without 

 artificial protection of any sort. The temperature of the soil is probably 



