13 2 SEEDS ALSO GERMINATE IN RED, YELLOW, AND ULUE LIGHT. 



index of any action of the absorbing medium. Some years ago, I had occasion to no- 

 tice that rapidity of growth was greatly influenced by the quantity of aqueous gas in 

 the atmosphere. Whether the observation possesses any novelty, I am not prepared 

 to say ; but if any one causes plants to grow in glass vessels containing the maximum 

 quantity of vapour which the atmosphere can hold at the temperatures under trial, their 

 unusual increase in dimensions will present a strikingly remarkable phenomenon. 



.503. In fourteen days from the commencement of this experiment another examina- 

 tion was made. 



No. 1, all its leaves of a grass green. 



No. 2, of a darker green. 



No. 3, green, but of a bluish tint when compared with a plant taken from the garden. 



No. 4, of a bright green. 



No. 5, pale whitish yellow, with no fresh leaves, but grown to thirteen times its for- 

 mer height, and apparently in a vigorous condition. 



With respect to No. 4, the plant under sulphocyanate of iron, I was not aware, at 

 the time of making this trial, of the singular properties of that substance in relation 

 to light ; in the course of a fortnight, which had elapsed, the solution, from being of a 

 deep blood red, had become perfectly colourless. No reliance is, therefore, to be placed 

 on this result. 



504. Among a number of experiments which were instituted with an intention of 

 illustrating the same point, and which gave analogous results, it may be mentioned that 

 the seeds of common garden cress were caused to germinate and grow in the boxes 

 mentioned above ; and no matter what was the substance through which the light 

 passed, the young plants, after reaching a certain size, were always green, but those 

 which grew in the dark had yellow leaves and white stalks. 



505. The general result of these trials goes to prove that it is not this or that species 

 of ray which gives rise to the colour of leaves ; the absence of the chemical ray, or 

 of the calorific ray, does not appear to affect it, nor have we any direct proof that the 

 calorific ray exercises any influence. HUMBOLDT has stated that in the mines of Ger- 

 many, plants, as the poa annua, et compressa, plantago lanceolata, &c., grow in recesses 

 where the sun's light never comes, and, provided hydrogen gas be present, their colour 

 is green. In the Atlantic Ocean he saw a marine plant, fucus vitifolius, brought up from 

 the depth of 190 French feet, where, according to the calculations of BOUGUER, the 

 light was only equal to that emitted from a candle at 203 feet distance, and yet its 

 colour was green. DECANDOLLE mentions that artificial light, as that of lamps, gives 

 the same result ; a proof that it is certainly not the chemical, and, perhaps, not the 

 calorific rays, which cause the phenomenon. 



506. Perhaps light, in this case, acts only as a kind of stimulus ; it would be desirable 

 to make trial of some plants whose leaves are naturally white ; of this class there are 

 several individuals ; would they or would they not cause the decomposition of carbonic 

 acid I From many indications, it is not improbable that there is a variety of chemi- 

 cal rays, each of which brings about changes of a character appropriate to itself. As 

 yet we have not learned to distinguish these from each other, and are not provided with 



