PLANTS GROW IN LiGHTS OF VARIOUS COLOURS. 



499. To a deposite of some compound, in which carbon enters as an ingredient, we 

 are to refer the green colour of leaves, but not to carbon itself. The earlier chemists, 

 who did not possess those extremely delicate methods of gas analysis which are now 

 available, misunderstood this matter. They stated that, on exposing a plant to the sun- 

 shine, in contact with carbonic acid, the carbon was separated in a concrete state, the 

 oxygen being left, but such is not the fact ; by no known laws can such a change be 

 brought about, and hence any reasoning based upon it, as to the colour of plants, is ir- 

 relevant. For when a plant exposed to the sun decomposes carbonic acid, a certain 

 volume of oxygen disappears at the same time ; in lieu of this, and in obedience to the 

 laws which guide the transit of gases through tissues, an equivalent volume of nitrogen 

 is surrendered by the plant in return. Sometimes it is carbonic oxide which is absorbed, 

 sometimes oxalic acid, or other compound of carbon with less proportion of oxygen. I 

 do not here indicate from whence that nitrogen is derived, since botanists assert that 

 some plants contain no nitrogen at all; it may, however, exist in their juices as gas ex- 

 ists in spring water, or may be retained in a compressed state on their surfaces ; it is, 

 however, a remarkable fact that nitrogen is always present. 



500. The carbon thus taken from the acid does not pass through the tissue of the 

 leaf in a concrete form, or give rise to a concrete deposite; it bears with it a certain part 

 of the oxygen with which it was formerly united, the rest being set free ; the carbon 

 and oxygen so conveyed into the plant, entering into combination with hydrogen, give 

 rise to the chromule verte; hence we see that the green colour depends indirectly on 

 the decomposing action; that when this goes on without interruption, that is fully de- 

 veloped. 



501. I took five pea plants out of the garden, as nearly resembling each other in size 

 and other particulars as might be: they had just appeared above the surface of the 

 earth, and were beginning to put out leaves. These plants I designate by the numbers 

 1, 2, 3, 4, 5. Each one was planted in a small glass vessel with a hole in the bottom, 

 for the purpose of supplying it with water, after the manner of a common flower-pot. 

 Number 1 was placed in a box into which light passed which had traversed a solution 

 of sulphate of copper and ammonia. No. 2, in a similar box, into which light was ad- 

 mitted after having undergone the action of chromate of potassa. No. 3 was placed in 

 the open air. No. 4, in a box into which light had passed which had been transmitted 

 through sulphocyanate of iron. No. 5 was shut up in a dark closet. This arrange- 

 ment was completed on the second day of May. With a pair of compasses the height 

 of each plant was ascertained, and of that and of the number of leaves a memorandum 

 was taken. In three days' time an examination was made. 



No. 1 had attained three times its former height, and doubled its number of leaves. 

 No. 2, not quite twice its former height, no new leaves, in appearance not so plump 

 and transparent as formerly. 



No. 3, twice its former size, with no fresh leaves. 



No. 4, four and a half times its former size, and double its number of leaves. 



No. 5, three and a half times its former size; the leaves looked yellowish. 



502. It is here proper to remark, that the increase of size is not to be taken as an 



