110 FEEEZING. 



matter, are each diminished ; and the power of flowering and 

 fruiting is lost. If the unhealthiness of the plant is not so 

 great as to prevent the production of flowers, stiU they may not 

 expand, as often happens to double roses in cold summers in 

 England; or, if the flowers do unfold, the fertilising power of 

 the pollen is impaired or destroyed, and no production of seed 

 takes place. 



That the absence of healthy colour is sometimes owiug to 

 low temperature is certain. But the cause of the formation of 

 different colours in different plants is too obscure a subject to 

 suit the purpose of this work. It is, however, as well to observe 

 that the effect of decomposing carbonic acid and exhaling 

 oxygen is the production of a green colour, the intensity of 

 which is, in general, in proportion to the decomposing cause, 

 that is to say, to light : but that, if from any circumstances 

 water is not given off, but is retained in the system and allowed 

 to accTunulate, the green colour is altered and changes to 

 yellow ; as if the vegetable blue, which must exist in combi- 

 nation with yellow in order to form green, were discharged. 

 Such, indeed, is Macquart's explanation of the phenomenon ; 

 and it appears most conformable to theory and fact. It may be 

 regarded as incontestable, that among the most efficient means 

 of securing intense colo'urs is free exposure to air as well as 

 light. Experience tells us that, all other circumstances being 

 equal, flowers, fruit, or leaves produced without artificial 

 covering and at a moderate temperature, are much deeper in 

 colour than those which are developed under glass, in highly 

 heated buildings, where fresh air has little access. 



Should the temperature be so much lowered as to result in 

 freezing, a destruction of some plants and injury to others takes 

 place, owing to physical causes quite different from those whose 

 operation has been explained in the last paragraph. In what 

 degree frost acts upon the vegetable fabric depends upon the 

 specific nature of a plant, the least frost destroying some 

 species, while others, under equal circumstances, endure any 

 known amount of natural cold. 



The maimer in wMoli cold acts upon plants is one of the problems 

 wMoli have never yet been solved, and probably never wUl be. We see 



