Some curious experiments on this subject were made by Decandolle. He kept certain plants in two cellars, one warmed by a stove and dark, the other lighted by lamps. On some of the plants the artificial light appeared to have no influence, (_convolvulus arvensis_, _convolvulus cneorum_, _silene fruticosa_) and they still followed the clock hours in their opening and closing. The night-blowing plants appeared somewhat disturbed, both by perpetual light and perpetual darkness. In either condition they accelerated their _going_ so much, that in three days they had gained half a day, and thus exchanged night for day as their time of opening. Other flowers _went slower_ in the artificial light (_convolvulus purpureus_.) In like manner those plants which fold and unfold their leaves were variously affected by this mode of treatment. The _oxalis stricta_ and _oxalis incarnata_ kept their habits, without regarding either artificial light or heat. The _mimosa leucocephala_ folded and unfolded at the usual times, whether in light or in darkness, but the folding up was not so complete as in the open air. The _mimosa pudica_ (sensitive plant,) kept in darkness during the day time, and illuminated during the night, had in three days accommodated herself to the artificial state, opening in the evening, and closing in the morning; restored to the open air, she recovered her usual habits. Tropical plants in general, as is remarked by our gardeners, suffer from the length of our summer daylight; and it has been found necessary to shade them during a certain part of the day. It is clear from these facts, that there is a diurnal period belonging to the constitution of vegetables; though the succession of functions depends in part on external stimulants, as light and heat, their periodical character is a result of the structure of the plant; and this structure is such, that the length of the period, under the common influences to which plants are exposed, coincides with the astronomical day. The power of accommodation which vegetables possess in this respect, is far from being such as either to leave the existence of this periodical constitution doubtful, or to entitle us to suppose that the day might be considerably lengthened or shortened without injury to the vegetable kingdom. Here then we have an adaptation between the structure of plants, and the periodical order of light and darkness which arises from the earth’s rotation; and the arbitrary quantity, the length of the cycle of the physiological and of the astronomical fact, is the same. Can this have occurred any otherwise than by an intentional adjustment?