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25. With regard to light, we may observe, that 

 whatever doubts may exist concerning its effects on 

 the germination of seeds, there can be no question 

 but that it exerts a peculiar agency on the appear- 

 ance and qualities of plants. M. du.Fay is said to 

 have been the first philosopher who remarked how 

 much the green colour of plants depended on the 

 agency of light ; and from an observation made 

 about the year 1760, the late Professor Robison of 

 Edinburgh was led to infer, that it was very actively 

 employed in giving rise, not only to the colour, but 

 to the odour and combustibility of vegetables. Ha- 

 ving gone down into a coal-pit in the neighbour- 

 hood of Glasgow, he brought up some whitish-look- 

 ing plants, but no one knew what they were. After 

 being exposed to the light, however, the white leaves 

 died away, and were succeeded by green buds ; and 

 the plant acquired the smell of tansy. On further 

 inquiry, he found, that the sods on which the plant 

 grew had been taken down into the pit from the 

 part of an adjoining garden ; and although the plant 

 had continued to grow in its new situation, yet nei- 

 ther in colour, in odour, nor combustibility, did it 

 at all resemble plants of the same species which had 

 vegetated under an exposure to light. The etiola- 

 tion or blanching of the roots of celery, and of the 

 inner parts of cabbages and lettuce, are familiar ex- 

 amples of the same kind ; and Mr Davy ascertained, 

 that after the green colour is formed, it will again 

 disappear, if the plant be excluded from light. The 

 leaves of the common lettuce were in six days ren- 

 dered very pale, by being deprived of light, and at 

 the end of another week they were quite white : the 



