182 ON THE MODE OF ACTION OF HEAT ON PLANTS. 



different intensities of sunshine, according to seasons and geo- 

 graphical positions. For this purpose I have proceeded as 

 follows, by way of trial, hoping in future to do better, or that 

 others may do better. 



I have chosen some annual plants, of which the periods of 

 flowering and ripening appeared well marked, and which seemed 

 to be capable of vegetation at temperatures very litlle above 0°. 

 I have sown them simultaneously in the sun and in the shade. 

 I have sown the same species, in the sun, at different periods, 

 from spring onwards. I have noted precisely the periods of 

 flowering and ripening ; I have compared them with thermo- 

 metrical means obtained in the ordinary manner by observations 

 in the shade, and the result lias been an exact estimate of the 

 surplus heat received by certain plants under the direct influence 

 of the sun — an estimate given in the form of a certain number of 

 days having a certain mean temperature in the shade. An 

 example will better explain the process. 



Some Cress (Lepidium sativum) was sown on the same day, 

 the 24th May, 1847, in a bed in the shade, and 'in another 

 exposed to the sun, in the Botanical Garden of Geneva. The 

 seeds germinated rapidly, as is usual with this little species. 

 The plants in the shade flowered on the 13th July, and ripened 

 the 17th August. Let us adhere to the latter date, in order to 

 consider the aggregate life of the plant. From the 24th May to 

 the 17th August are 85 days. The mean temperature of 

 Geneva, according to the usual observations taken at the Ob- 

 servatory with a thermometer in the shade, was 17° 24. The 

 product of 85 by 17° 24=1465, expresses, according to M. 

 Boussingault's method, the aggregate heat required by the plant 

 to germinate and ripen its seeds. There is no error here : for 

 the plant, like the thermometer, was in the shade, and no nega- 

 tive quantities were taken into account in the means. The 

 plants exposed to the sun flowered the 12th July, and ripened 

 the 9th August -, total, from the period of sowing to ripening, 77 

 days. The mean temperature during those days, measured by a 

 thermometer in the shade, was 17° 06. The multiplication of 

 77 by 17° 06 gives only 1313 : thus, a heat apparently of 1313° 

 has produced the same effect as 1465° in the former case. Yet 

 the same aggregate heat on the same species cannot produce two 

 such different effects. It is clear that the plants in the sun- 

 shine have received 1313°, measured by the thermometer in the 

 shade, plus an additional quantity resulting from the action of 

 the sun's rays — an additional quantity not recorded by the ther- 

 mometer of the Observatory, and represented by the difference 

 between 1313 and 1465=152°. In other terms, the effect, 

 calorific or chemical, of the direct rays of the sun was equal to 

 152° of an ordinary thermometer in the shade. This effect was 



