BLANCHING. 33 



curious to know why blanching a plant should destroy 

 its unwholesomeness, and therefore we will aoain 

 digress from our principal subject for the purpose of 

 explaining this curious fact. 



In my last letter, I told you that the business of 

 leaves is to expose to light and air the sap they suck 

 out of the stem. The consequence of light and air 

 acting upon the surface of leaves, is the forming 

 in their substance, which is originally of the same 

 yellowish-white that you see in seeds, a green colour, 

 which is more or less deep in proportion to the 

 degree in which the light is powerful ; thus a plant 

 which stands exposed to the sun all day long, has its 

 leaves of a darker green than another which grows 

 among other trees, or near a building which throws 

 it into the shade for a part of the day : and the latter 

 again is darker green than a plant which grows 

 at the north side of a high wall, or in an enclosed 

 court which the sun's rays never enter. In like 

 manner, if you cause a plant or any part of a plant 

 to grow in total darkness, it will be entirely desti- 

 tute of greenness; or in other words, the substance 

 of the plant will remain of its original yellowish- 

 white, because no green matter can be formed 

 but by the action of light ; and if a part already 

 green is kept for a long time in darkness, it will be- 

 come yellowish-white, in consequence of all its green 

 being destroyed by the peculiar action of the atmo- 

 sphere upon plants in darkness. — This is the explana- 

 tion of blanching. But mere loss of colour is not the 

 only consequence of plants being kept in the dark } 



D 



