of the Organic Systems of Vegetables. 93 



they had severally gained from three to five grains each, which 

 increase (which was evident from their succulent appearance 

 also) could only have taken place by absorption through the 

 cuticle, as their cut petioles were, as in the previous experi- 

 ments, defended by soft cement. 



That the exhalation of leaves is very great was formerly 

 shewn by Hales, in his Vegetable Statics, but his mode of 

 experimenting was extremely inconvenient : I have found a 

 much simpler means to answer better. It is to put the leaves 

 or cuttings, or plant to be experimented on, into a glass vessel, 

 graduated so that the quantity of water which is put in may 

 be accurately known, and the quantity that is lost ascertained 

 by the variation in its height. The surface of the water should 

 then be covered with a stratum of oil about half an inch thick, 

 which will closely invest the stalks and prevent any evaporation 

 from the surface of the water ; so that all which is lost must 

 have been perspired by or be contained in the plant. If a 

 growing vegetable in a pot be the subject of experiment, it 

 will merely require to be set in a larger vase, and to have the 

 water above the edge of the pot so that the oil may inclose and 

 cover the whole*. By experiments so conducted, I have 

 found that a single leaf of the common sunflower (Helianthus 

 annuus), weighing only thirty-one grains and a half, absorbed 

 in four hours twenty-five grains of water ; the leaf had in- 

 creased in weight only four grains and a half, so that twenty 

 grains and a half of water had disappeared, the greater part 

 of which had been exhaled, as was proved by other similar 

 experiments, in which the apparatus was placed under a re- 

 ceiver and the vapours condensed upon its sides. These 

 details might be multiplied, but the above are sufficient for 

 our present purpose. 



The experiments, however, with the leaves of plants, which 

 have excited the most attention, are those that have shewn the 

 changes they effect in the constitution of* confined portions of 

 atmospheric air ; which changes are of two directly opposite 

 kinds, viz., its deterioration and its amelioration, i. e., the 

 increase and the diminution of the oxygen it contains. Both 



* To prove the power of oil iu preventing the evaporation of water, I have kept 

 two ounces of water in an open graduated measure, covered only by a very thin 

 stratum of oil, for upwards of two years, without any sensible diminution. 



