Oberon is right, it's from Jonathan Swift:
Gullivers Travels, Part Three, Chapter V;
published in “Everyman´s Library” by J. M. Dent & Sons LTD
London, 1940, reprint 1960, text taken from Faulkner´s
editions of 1735-8.
I find the ida of extracting sunlight from cucumbers scientifically .. challenging.
Think of what would be possible, with sunflowers .. 
And a good argument to p. off notorious environmentalists 