20 DISEASE IN PLANTS. 



the interesting result that the agricultural or 

 forest plant is adapted to catch and retain, broadly 

 speaking, just those particular rays which possess 

 most energy. 



The probability is increasing that the protoplas- 

 mic machinery is the really effective mechanism in 

 the process, and we may figure this machinery as 

 so holding or presenting the molecules of carbon- 

 dioxide and water to the impact of the light- 

 vibrations, that the latter are enabled to undo 

 the molecular structure ; the atomic combinations 

 thereby liberated may then be supposed to form 

 a body like formic-aldehyde, which by polymerisa- 

 tion becomes a carbohydrate of the nature of a 

 sugar such as glucose, which the protoplasm then 

 builds up into its substance and subsequently 

 deposits as starch, and stores temporarily in the 

 form of grains or as amorphous material. 



This is partly hypothetical, and is largely due 

 to the careful deductions of the chemists, but there 

 are very many facts now to hand which bear out 

 its probability, especially the recent advances in 

 our knowledge of the sugars, and the experimental 

 feeding of leaves and plants deprived of starch 

 with such substances as dextrose, levulose, maltose, 

 and other sugars, as well as glycerine and other 

 bodies which should be convertible into, or yield 

 them, if the theory is true. In this last connec- 

 tion, the careful and extensive experiments of 

 Acton, A. Meyer, Boehm, and Laurent should 

 be mentioned. It would be interesting to enlarge 

 upon Engelmann's beautiful physiological experi- 



