DESIGN. 33 



almost, if not quite, impossible to form pure colors artificially. 

 The yellow, for instance, of the bichromate of potass contains 

 both red and violet in abundance."^ 



It has been already ascertained that the amount of assimila- 

 tion, and consequently of the healthy exercise of its vital func- 

 tions, depend upon the intensity of the light to which the plant 

 is exposed. In bright sunshine they perspire most; in weak 

 diffused light, and in darkness, none at all. ' Hales found that a 

 cabbage lost nineteen ounces of weight per diem, and a sunflower 

 twenty. He estimated the average rate of perspiration by plants 

 to be equal to seventeen times that of a man. In one of his exper- 

 iments he found that the branch of an apple-tree, two feet long, 

 with twenty apples, exposed to bright sunshine, raised a column 

 of mercury twelve inches in seven minutes. But a dry, arid 

 atmosphere, especially if in motion, also robs the plants of their 

 moisture independently of light. 



The clear and unclouded skies of this country do not, as some 

 suppose, obviate the necessity of surrounding the plant with, a 

 transparent medium in all directions, nor does the dark and sun- 

 less climate of England render it necessary that the houses 

 should be more transparent there than here. It is a practical 

 absurdity to fancy that in England there is less light than in 

 this country, and that, because the mid-day sun is more powerful, 

 they can do with a greater opacity of structure. Those who 

 make such statements manifestly know as little of the climate 

 of England as of the natures of its skies, and mislead those who 

 know as little as themselves. No argument whatever, based 

 upon the brightness of the sunshine at mid-day, can serve to 

 justify the adoption of one single inch of opaque material in a 

 horticultural building. It is very easy to reduce the quantity of 

 light, or break the rays of the sunshine, by shading; but it is not 

 so easy to increase the quantity of light in the dark and gloomy 

 months of winter ; and such sort of plant-houses will damp the 

 energies and zeal of the most skilful gardener, as well as his 

 tender exotics. When he sees these errors, which, he cannot 

 remedy, and observes his plants speaking in a language which 



♦ For further experiments on Light, see Sect. IV., GIm%. 



