BOTANY. 



ffgetxWe sleep of plants displays itself differently in dif 

 1 ">'Y- plants, that is to s:iy, according to the an .ii: ; ,en;ent 



- T ~<~' ' ,,( the leaves ; so that in some they are applied to 

 each other, in others to the stem, and so on, accord- 

 to the peculiarities of the respective plants. 

 Mirbel has been at the pains of clarifying the modes 

 of the sleep of plants, and he has certainly succeeded 

 in giving a very important aspect to the enunciation 

 of a simple fact. 



Such is a rapid sketch of the principal points 

 known with respect to this phenomenon : little is 

 to be learned from books respecting it, and much 

 from nature. It is therefore not a little astonishing, 

 that no philosopher, possessing opportunities and 

 leisure, should have prosecuted the inquiry still far- 



fpothcses. ther than it has hitherto been. Notwithstanding 

 the want of facts, solutions of the difficulties have 

 abounded. Bonnet's hypothesis has been extend- 

 ed to explain it ; and many other suppositions, 

 equally unsatisfactory, have been advanced. Lin- 

 naeus thought that it was induced by the absence of 

 light. Hill pushed this opinion still further, and en- 

 deavoured to establish it by some plausible experi- 

 ments. Decandolle has still more recently made some 

 experiments on the Mimosa pudica, which render the 

 opinion probable : He lighted up a dark cave 

 with lamps, in which he had placed the plant at 

 midnight ; the leaves expanded, and in the following 

 day they closed, on extinguishing the light. He did 

 not succeed in his attempts to induce the same re- 

 sults in other plants of an equally irritable habit. 

 This folding up of the leaves, or sleep, appears to be 

 the result of certain actions of other causes on the 

 irritability of the plant, and among these causes light 

 appears to maintain a very high rank. The various 

 living phenomena of every vegetable give force to 

 these suppositions ; but the mode of action is be- 

 yond our reach : we must be satisfied with learning 

 the general laws by which nature conducts her ope- 

 rations. 



fthe As the fall of the leaf is intimately connected 

 with the state of the irritability of the plant, we 

 shall detail the leading facts that have been ascer- 

 tained respecting it in this place. It is familiar to 

 all, that, about the middle of autumn, the leaves of 

 all annual and of many perennial plants, gradually 

 lose their vigour, change their colour, and having 

 their vital powers completely exhausted, at length 

 are separated from the parent branch. The singular 

 variety of colour exhibited in a grove about the end 

 of autumn, constitutes one of the splendid objects of 

 an autumnal landscape. Many trees do not shed 

 their leaves at the usual season : among these is the 

 oak, in which this event does not take place until the 

 spring ; others, again, which are called evergreens, 

 preserve their foliage throughout the whole year ; a 

 fact which Mirbel has endeavoured to explain, by 

 first supposing that the fall of the leaf, under ordi- 

 nary circumstances, depends on the retention of 

 carbonic acid and water ; he then supposes that the 

 leaves of most evergreens contain resin, and that this 

 Tesiii having a peculiar affinity for the constituents of 

 water and carbonic acid, absorbs them, and thus pre- 

 vents their evil effects. 



In general, the trees that push forth their leaves 



very early, lose them proportionally toon. There 

 an'i however, exceptions to thin rule, us in the C 

 of the i'Jdcr, though one of the earliebt in producing, v 

 is one of the latest in losing them. 



This phenomenon is supposed by Dr Smith to be FiqiUna- 

 a mere sloughing of diseased or worn-out parts : lion, 

 an hypothesis so simple, and containing so nearly 

 the simple expression of the fact, that few will con- 

 tend against it. 'Mirbel has given a more laboured 

 explanation, which appears to us to blend, without 

 sufficiently marking the boundaries, facts with opi- 

 nions, an error that cannot be too sedulously avoid- 

 ed in every species of physiological research. Ac- 

 cording to this physiologist, there is a very free 

 communication between the vessels of the leaf and 

 those of the liber, so that fluids pass from each of 

 these into the others without difficulty, and the loot- 

 stalk is very firmly attached to the bark ; but a bud 

 is formed at the base of the foot- stalk, the liber be- 

 comes denser, the foot-stalk is protruded from its 

 situation by the growing bud, the vessels become 

 hard, the leaf ceases to receive fluids, and to expire. 

 It then falls off, and undergoes decomposition. 



A strong analogy has been supposed to exist be- Analogy 

 tween the leaves of trees and their roots, because between 

 both of them take up fluids, and give out certain other ***** 

 Huids. Some have fancied the analogy to be so com- 

 plete, as to authorise leaves being called tierial roots ; 

 an idea too whimsical to be entertained for a moment. 



It is a certain fact, that the leaves of plants Absorpi 

 will both receive and give out aqueous fluid, tion. 

 The experiments of Bonnet are among the most sa- 

 tisfactory on this subject. By a very complete se- 

 ries of experiments, he determined the relative vi- 

 gour with which this function is carried by the two 

 broad surfaces of each leaf, and also the relative vi- 

 gour with which it is carried on in dial-rent kinds of 

 leaves ; and it is curious to examine the results with 

 which he has presented the world. His experiments 

 were made on the leaves of fourteen herbaceous 

 plants, and on those of sixteen trees, which were 

 laid upon the surface of water. Six of the former, 

 Arummuculatum, Phaseoh/s vulgaris, the sun- flower, 

 spinach, and the small mallow, (probably Mnlva ro- 

 tundifolia) lived equally well when either surface 

 was applied to the water. Six others, plantain, white 

 mullein, the great mallow, (probably M. syhestris), 

 the nettle, cockscomb, and purple -leaved amaranth, 

 lived longest wkh their upper surface to the water. 

 The proportion of the time which each of these 

 leaves lived with the different surfaces exposed, va- 

 ried much in the different species. The Marvel of 

 Peru, and Balm, were the two remaining herbaceous 

 plants on which Bonnet made his experiments, and 

 they throve with their upper surfaces on the water. 

 Of the leaves of the trees, those of the Lilac and As- 

 pen lived equally well in all situations ; all the 

 others, but most remarkably those of the white mul- 

 berry tree, succeeded decidedly best with their under 

 surfaces laid on the water. The vine, the poplar, 

 and the walnut, died when laid on their upper sur- 

 faces in water, as soon as when altogether deprived 

 of that fluid. It is very evident from the above facts, 

 that leaves are furnished with organs thdt absorb 

 fluids necessary for their support ; and we learn from. 



