ON THE MODE OF ACTION OF HEAT ON PLANTS 



185 



tions at Geneva give us an exact measure. The species sown on 

 the 24th May were less affected by the sun up to their period of 

 flowering than those sown on the 23rd April (0° 4. or 0° 8 per 

 day, and the latter 2° 2 and 3" 8). But it happened that the 

 month of June. 1847, was more cloudy than the month of May. 

 The plants sown the 23rd April made their chief progress to- 

 wards flowering during May. In this month the sky was 

 covered with clouds by 0*41 of its extent at twelve o'clock.* and 

 from the 17th to the 20th June, the most important period for the 

 plants, it was covered by 0*47 of its extent. The plants sown 

 the 24th May made their chief progress towards flowering 

 during June and the first week of July, a period in which 

 the sky was clouded at twelve o'clock by 0*52 of its extent. To 

 determine the effects of nebulosity with precision, it would be 

 necessary to take it into account every day with relation to the 

 temperature of the day. AYe should also know the effect of 

 watery vapours, which intercept heat although the air may retain 

 its transparence. AVe cannot enter into all these details, but it 

 is evident that, in order to estimate the solar action in different 

 countries, or in different months in the same country, we must, 

 as much as possible, take into account either the mean extent of 

 clouds, or at any rate the number of clouded days. According 

 to observations made at Geneva, the mean nebulosity varies but 

 little from one year to another. Thus this element, carefully ob- 

 served, misrht serve to characterise climates with facilitv and 

 regularity. 



I tried fresh experiments in 1849, making use of Rape (Colza). 

 This species, however, did not answer my expectations in regard 

 to precision in the time of flowering and ripening. Besides which 

 I was not satisfied with the situation afforded by the Botanical 

 Garden of Geneva.j The latter motive, as well as the hope of 

 establishing a comparison between the solar action at Geneva and 

 in a more southern climate, induced me to request my friend, 

 M. Moquin-Taudon, professor of botany at Toulouse, to try in 

 the latter town a series of experiments. He very kindly acceded 

 to my request. He even extended his sphere of observations 

 to several species. Knowing well bis habitual exactness, and 

 also that he had very favourable localities at Toulouse, I hoped 

 to have obtained interesting results from his labours. Unfortu- 



* The observations made at Geneva indicate the proportion of the firma- 

 ment occupied by clouds. Estimated in decimals, this proportion gives 

 means easy of calculation. This practice is far preferable to the terms 

 cloudy, slightly clouded, &c, ordinarily used in meteorological tables. 



f The shade was not perfect. It is difficult to procure good shade so as 

 to leave at the sanie time to the plants full daylight, which they caunot do 

 without. 



