ON THE INFLUENCE OP SOLAR RADIATION ON PLANTS. 1^ 



those under the blue being somewhat the longest. Presently the effect of 

 the yellow light in causing the production of very long roots began to show 

 itself. All the seeds germinated. On July 19th, the peas were treated as 

 the wheat had been. 



The peas that were sown in mould began to grow equally at first, but in 

 about three weeks' time those under the colourless glass were the shortest. 

 They grew luxuriantly and filled the bell-glasses, but at the beginning of 

 July the pea-plants which grew under the blue shade, and which had never 

 thriven, shrivelled and died away. The leaves never opened properly. The 

 following were the numerical observations made July 19th: — 



On comparing these last results, it is evident that whether with or without 

 mould, the peas that grew under the blue glass display an inferiorit\\ The 

 peas growing in mould certainly produced the most healthy plants when they 

 were exposed to all the influences of the solar ray, and the deprivation of the 

 luminous principle proved fatal to them in their more mature growth, although 

 the removal of the chemical ray had little effect. 



These experiments indicate no relative difference in the actions of the 

 three different coloured lights upon the germination of seeds, dependent on 

 the absence or presence of soil ; and they afford further confirmation of mv 

 former view, that the chemical rays rather militate against than favour the 

 healthy germination of at least these particular instances of Monocotyledonous 

 and Dicotyledonous plants. I remain unacquainted with the reason why the 

 experiments of some other observers, arid indeed one or two of my own, ex- 

 hibit a tendency of seeds to germinate more readily under a blue glass. It 

 may be from the more complete darkness thus produced ; but the problem is 

 evidently a difficult and intricate one, and I abstain from further conjecture. 



Among the queries at the close of the last Report was the following :•— 

 "Does carbonic acid act specifically in the prevention of germination, or 

 merely by the exclusion of oxygen ? " It was thought that this might be 

 determined by substituting that gas for the nitrogen in the air, and observing 

 whether seeds germinated equally well in such an atmosphere. Experiments 

 previously recorded rendered it unnecessary for me to satisfy myself again 

 that peas and wheat would commence growing in a colourless jar of twenty- 

 five cubic inches capacity. Such ajar was therefore filled with a mixture of 

 four parts of carbonic acid and one part of oxygen, placed over mercury, on 



1855. C 



