42 LIGHT, CONSIDERED WITH 



Light, considered with reference to Horticulture. 



Light is one of the most important agents in the growth of plants. 

 It strengthens the colour and matures the growth of both wood and 

 fruit. When plants are grown in the dark they are blanched white, and 

 become an inodorous, insipid mass that can yield no flame when 

 burned. 



Plants suffer most at a distance from light, when the light is only 

 from the top, or one-sided. This has been called the attraction of light, 

 but is no explanation. In the one-sided light it may be the greater 

 solidifying of the side next the light which draws. In the top-light of 

 frames, the want of direct light at the sides may cause partly the 

 greater elongation of the top ; but plants elongate below glass, even 

 though surrounded by light. The want of motion is a great cause of 

 this : plants uniformly elongate more in a sheltered than in an exposed 

 place. If there is any such thing as attraction between light and plants, 

 like that which determines roots to follow the direction of their food, 

 it will be, like the attraction of gravitation, more easily perceivable 

 in its effects than capable of explanation. Refraction will disperse 

 the light : it is difficult to understand how it should weaken what does 

 pass through. The chemical power of light, however, is so much con- 

 nected with electricity, that it may be weakened in a way we cannot 

 account for. The chemical power of light is greatest in the least 

 luminous part of the rays ; and yet, as the quantity of light is equal, 

 that at the equator must have most power. There is a connexion 

 between heat, light, and electricity, not yet explained. The optical 

 qualities of light have been much more attended to than the chemical. 

 The red rays are less refracted than the blue ; thus causing the 

 red of the rising and setting sun, and the grey blue of the dawn. Per- 

 haps more of the blue or chemical portion of the sun's rays may thus 

 be lost in refraction. The carbon contained in all plants, and which 

 of course is in greatest abundance in such as have woody stems, is 

 entirely the result of the action of light on the leaves, by which plants 

 are enabled to decompose carbonic acid, liberate oxygen and appro- 

 priate the carbon of the atmosphere to themselves. Fruits before they 

 are ripe are acid ; that is, their hydrogen and carbon are combined 

 with an excess of oxygen ; but they are rendered saccharine by the 

 action of light, which occasions the evolution of the oxygen, and the 

 fixation of carbon, by which the vegetable acid is converted into 

 sugar. In a word, no plant, nor any part of a plant, can be brought 

 to perfection without light ; but it deserves also to be remarked that, 

 in the cultivation of plants for the use of man, it is sometimes not 

 desirable to bring all the parts of a plant to perfection ; and, in these 

 cases, the absence of light is as necessary as its presence is in others. 

 For example, in the case of the Celery and other plants, the stalks of 

 these, when rendered green by light, are disagreeable to the taste and 

 even poisonous, but by excluding the light, they are rendered 

 wholesome and agreeable : the same may be said of the tubers of the 

 Potato, and of the stalks and leaves of Cardoons, Endive, &c, 



