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ON THE MODE OF ACTION OF HEAT ON PLANTS. 183 



distributed over 77 days, and was therefore equivalent to 1° 97 

 (very near 2°) per day. 



1 might, in order to simplify the calculation of the mean 

 temperatures, have considered only the eight additional days 

 required by the plants in the shade to ripen their seeds. During 

 these eight days the mean temperature, multiplied by 8, would 

 have given 152°; thus expressing, in degrees of a thermometer 

 in the shade, the value of that which the plants in the shade 

 have received the less, or those in the sun the more. I have 

 preferred calculating the figures for the whole life of the plant, 

 in order the better to estimate the gradual effect of solar action 

 and of general temperature. 



Seeds of Iberis amara were sown on the 23rd April, 1847, in 

 the shade and in sunshine. The plants in the shade flowered the 

 28th June, and ripened the 9th September. Those in the sun 

 flowered the 20th June, and ripened the 1 1 th August. Consi- 

 dering the whole period, the first required 139 days and the 

 second 110 days to complete the cycle of their vegetation as 

 annual plants. The plants in the shade received, in degrees of 

 thermometer observed in the shade, an aggregate of 2219° (the 

 product of 139 days by their mean temperature) ; the plants in 

 the sun appeared to receive only 1754°, calculating the temper- 

 ature of their 110 days in degrees of a thermometer in the shade. 

 The difference, 465, expresses the effect of the solar rays, calcu- 

 lated in degrees of a thermometer in the shade. This amount 

 gives a mean of 4° 2 per day. 



Other species were sown at the same time and their progress 

 observed simultaneously, viz., Sinapis dissecta, Nigella sativa, 

 Iberis umbellata, and Linum usitatissimum. One of these, how- 

 ever, the Iberis umbellata, did not sufficiently show its period of 

 maturity for the results to be complete in its case. Without 

 entering into details, these several species gave the following 

 results as the effect of solar rays calculated per day, and expressed 

 in degrees of a thermometer in the shade : — 





From Sowing 

 to Flowering. 



From Flowering 

 to Ripening. 



From Sowing to 

 Ripening. 



Sowu on the 22nd of April :— 

 Iberis amara .... 

 Sinapis dissecta 

 Nigella sativa .... 



Sown on the 24th of May : — 

 Lepidium sativum . 

 Iberis umbellata . 

 Linum usitatissimum . 



- o *nJ 

 2*2 per day 

 3-8 



0-4 



O'S , , 



o 



6 • 5 per day 

 1-8 



4*7 



o 



4 • 2 per day 

 2-5 



4'6 ,, 

 2 f f 

 4-1 



