260 



the complement of the cloudiness and represents the relative duration 

 of sunshine, but owing to the varying altitude of the sun can by 

 itself alone give no idea of the intensity of the radiation received by 

 the plant. To obtain this ^ \st item and as no actinometric observa- 

 tions were made at Arras I give in the fifth colunni the results of 

 observations at Montsouris, expressed in actinometric degrees; 



The beets are reported to have sprouted very late and very un- 

 equally ; this was due not to dryness, since the rain during March and 

 April was in excess of its normal value, but was directly traceable to 

 the low temperature, which was especially low in April. 



The study of the development of sugar, week by week, as given in 

 the last two columns of the above table shows that after September 9 

 the sugar crop increased slowly, became stationary, and then fluctu- 

 ated very much as the weight of the leaves fluctuated. The rainfall 

 had at that time become light and the development of the beet seemed 

 to depend mostly on the temperature, so that it may be concluded that 

 the beet ceases to increase in its quantity of sugar after the mean 

 daily temperature falls below 13.1° C, and that there is no probable 

 advantage in leaving the beets in the soil after that date, which in 

 this case is September 29, 1879. 



Marie-Davy points out that the actual increase per decade of the 

 weight of the roots coincides with the increase of the rainfall and 

 the temperature, but the proportion of sugar increases with the degree 

 of radiation or total sunshine; the sunshine precedes the formation 

 of sugar, since its action is slow and indirect, being through the 

 assimilation that takes place within the leaves. It is therefore not 

 an excess of water, but a deficiency of light and heat that causes rainy 

 autumns and summers to give poor crops of sugar. Therefore, if 

 during dry, clear, warm summers having large radiation, one could 

 irrigate the fields properly one would realize the best conditions for 

 a good crop. Therefore, every ray of sunshine that strikes the 

 ground instead of the leaf is a loss to the formation of sugar and by 

 helping to evaporate the moisture of the soil it also causes further 

 great loss of sap to the plant. These conclusions agree with other 

 experiments made by Pagnoul, who raised beets both in darkness and 

 under a transparent bell glass, and again in the free air, and found 

 the amount of sugar to increase with the strength of the sunshine. 



The following table gives a general survey of the beet crops in Pas 

 de Calais and the corresponding climatic data at Montsouris, which 

 is about 90 miles south of Arras. The numbers given in the columns 

 for quantity and quality of the crops are the estimates obtained from 

 many planters and are recorded on the following scale : 1, very small 



