184 



ON THE MODE OF ACTION OF HEAT ON PLANTS. 



The diversity in these figures is not occasioned by any defect 

 in the method of obtaining them, but by the constant variations 

 in the solar action from one day to another according to the sea- 

 son and the nebulosity of the atmosphere. It is owinsr also to 

 the plants having been sown at two different periods, and to their 

 having each finished its life on a different day. Those which 

 lived chiefly in summer felt so much the more the effects of a 

 season when the sun has more force than in any other. 



The following is an instance of the increasing and afterwards 

 decreasing action of the sun on vegetables from spring to autumn. 

 It is taken from two species which unfortunately both proved 

 ill-adapted to fixing precisely the day of maturity. Nevertheless 

 the experiment is not useless. 



Flax (Linum usitatissimum) and Iberis amara were sown in 

 the Botanic Garden of Geneva in the sun. The sowings of Flax 

 marked A, B, C. and those of Iberis A. B. C. were in a border 

 which, besides the direct rays of the sun, received also the rever- 

 beration from a wall situated at the distance of a yard. The 

 sowings of Flax D and E were beyond the influence of the wall, 

 but still in the sun. The two latter were made in 1848, the 

 others in 1847. 





Date of Sowing. 



Date of 

 Maturity. 



Duration 

 in days. 



Mean Tem- 

 perature. 



Product. 







Flax A . . . 



April 23 . 



August 2 . 



101 



1 c £ 

 15-89 



1605 



Flax B . . . 



May 24 . . 



August 7 . 



75 



1696 



1272 



Flax C . . . 



June 24 . . 



Sept. 3 . . 



71 



17*70 



1257 



Flax D . 



April 29 . 



August 12 . 



105 



16-37 



1719 



Flax E . . . 



June 9 . . 



Sept. 7 . . 



90 



17-82 



1604 



Iberis A 



April 23 . 



August 1 1 . 



110 



15-95 



1754 



Iberis B 



May 24 . . 



Sept. 10. . 



109 



16-70 



1821 



Iberis C . . 



June 24 . . 



October 26 



124 



14-99 



1858 



It will be seen that the later the Flax was sown the smaller 

 was the product of the temperature by the number of days, be- 

 cause this species, ripening in summer, received an increasing 

 amount of additional solar heat — a heat not indicated by the ther- 

 mometer in the shade. The Iberis, on the contrary, showed 

 products increasing as the period of sowing was delayed, because 

 its vegetation only terminated in September or October, when 

 the, solar heat diminishes. 



In the result of my experiments of 1847, I was at first puzzled 

 by an anomaly, which however was very well explained bv the 

 state of nebulosity of the sky. of which fortunately our observa- 



