694 



FARMERS' REGISTER, 



[Na. 11 



late observations of Mr. Knight, that the direc- 

 tions of tendrils, and the spiral Ibrm they assume, 

 depend upon the unequal action of light upon 

 them, and a similar reason has been assigned by 

 M. De Candoile to account for the turning of the 

 parts of plants towards the sun; that ingenious 

 physiologist supposes that the fibres are shortened 

 by the chemical agency of the solar rays upon 

 them, and that, consequently, the parts will move 

 towards the light.* 



The leaves, the great sources of the permanent 

 beauty of vegetation, though infinitely diversified 

 in their forms, are in all cases similar in interior 

 organization, and per/brm the same fijnctions. 



The alburnum spreads itself from the foot-stalks 

 into the very extremity of the leaf; it retains its 

 vascular system and its living powers; and its pe- 

 culiar tubes, particularly the trachccE, may be dis- 

 tinctly seen in the leaff 



The green membranous substance may be con- 

 sidered as an extension ol' the parenchyma, and 

 the fine and tliin covering as the epidermis. Thus 

 the organization of the roots and branches may be 

 traced into the leaves, which present, however, a 

 more perfect, refined, and minute structure. 



One great use of the leaves is for the exposure 

 of the sap to the influence of the air, heat and 

 light. Their surface is extensive, the tubes and 

 cells very delicate, and their texture porous and 

 transparent. 



In the leaves much of the water of the sap is 

 evaporated; it is combined with new principles, 

 and filled lor its oTiTanizing functions^ and probably 

 passes, in its prepared slate, from the extreme tubes 

 of the alburnum into the ramifications of the cor- 

 tical tubes, and then descends through the bark. 



On the upper surface of leaves, which is exposed 

 to the sun, the epidermis is thick but transparent, 

 and is composed of matter possessed of little or- 

 ganization, which is either principally earthy, or 

 consists of some homogeneous chemical substance. 

 In the grasses it is partly siliceous, in the laurel 

 resinous, and in the maple and thorn it is princi- 

 pally constituted by a substance analogous to wax. 



By these arrangements any evaporation, except 

 from the appropriated tubes, is prevented. 



On the lower surface the epidermis is a thin 

 transparent membrane llill of cavities, and it is pro- 

 bably allogeiher by this surface that moisture and 

 the principles of the atmosphere necessary to vege- 

 tation are absorbed. 



If' a leaf be turned, so as to present its lower 

 surface to the sun, its fibres will twist so as to bring 

 it as much as possible into its original position ; 

 and all leaves elevate themselves on the foot-stalk 

 during their exposure to the solar light, and as it 

 were move towards the sun. 



* According to M. De Candoile, in the side of a 

 branch most exposed to light, more carbonic acid is 

 decomposed and more carbon is fixed in its tissue, 

 and conseciuently it sooner becomes firm and solid 

 than on the other side where least exposed to the 

 agent in question, and therefore the latter elongates in 

 a greater degree than the former. The curvature to- 

 wards the light is the result. Vide Phisiologie Vege- 

 tale, par M. De Candoile, p. 832. Paris, 1S.32.— J.'b. 



t Fig. 11. represents part of a leaf of a vine mag- 

 nified and cut, so as to exhibit the tracheas; it is co- 

 pied, as are also the preceding figures, from Grew's 

 Anatomy of Plants. 



This efiect seems, in a great measure, dependent 

 upon the mechanical and chemical agencv of light 

 and heat. Bonnet made artificial leaves, which, 

 when a moist sponge was held under the lower 

 surface, and a heated iron above the upper sur- 

 liice, turned exactly in the same manner as the na- 

 tural leaves. This, however, can be considered 

 only as a very rude imitation of the natural pro- 

 cess. 



What Linnfeus has called the sleepof the leaves, 

 appears to depend wholly upon the suspension of 

 the action of light and heat, and on the operation 

 of moisture. 



This singular but constant phenomenon had 

 never been scientifically observed, till the attention 

 of the botanist of Upsai was fortunately directed to 

 if. He was examining panicularly a species of 

 lotus, in which four flowers had appeared during 

 the day, and he missed two in the eveninsr ; by ac- 

 curate inspection, he soon discovered that these 

 two were hidden by the leaves, which had closed 

 round them. Such a circumstance could not be 

 lost upon so acute an observer. He immediately 

 took a lantern, went into his garden, and witnessed 

 a series of cuinous facts before unknown. All the 

 simple leaves of the plants he examined had an 

 arrangement totally different frona their arranga- 

 ment in the day : and the greater number of them 

 were seen closed or folded together. 



The sleep of leaves is, in some cases, capable of 

 being produced artificially. De Candoile made this 

 experiment on the sensitive plant. By confining it 

 in a dark place in the day-time, the leaves soon 

 closed; but on illurainatingthechamberwtth many 

 lamps, they again expanded. So sensible were 

 they to the effects of light and radiant heat. 



In the greater number of plants the leaves an- 

 nually decay, and are reproduced, their decay takes 

 place either at the conclusion of the summer, as in 

 very hot climates, when they are no longer supplied 

 with sap, in consequence of the dryness of the soil, 

 and the evaporating powers of heat ; or in the 

 autumn, as in the northern climates at the com- 

 mencement of the frosts. The leaves preserve 

 their functions, in common cases, no longer than 

 there is a circulation of fluids through them. In 

 the decay of the leaf, the color assumed seems 

 to depend upon the nature of the chemical change; 

 and as acids are generally developed, it is usually 

 either reddish-brown or yellow; yet there are great 

 varieties. Thus in the oak, it is bright brown; in 

 the beech, orange; in the elm, yellow; in the vine, 

 red ; in the sycamore, dark-brown ; in the cornel 

 tree, purple; and in the woodbine, blue. 



The cause of the preservation of the feaves of 

 evergreens through the winter is not accurately 

 known. From the experiments of Hales, it ap- 

 pears that the force of the sap is much less in 

 plants of this species, and probably there is a cer- 

 tain degree of motion in ir, in warm days, even in 

 winter; their juices are less watery than those of 

 other plants, and probably less liable to be con- 

 gealed by cold, and certainly not so easy of de- 

 composition ; and their vessels are defended by 

 stronger coatings from the action of the elements. 



The production of the other parts of the plant 

 takes place at the time the leaves are most vigor- 

 ously performing their functions. If the leaves 

 are stripped ofTlrom a tree in spring, it uniformly 

 dies ; and when many of the leaves of forest trees 

 are injured by blasts, or long-contmued dryness, 



