106 



SIR H. DAVY S AGRICULTURAL CHEMISTRY. 



On the u])per surface of the leaves, 

 which is ex])osed to the sun, the epidermis 

 is thick but transparent, and is composed 

 of matter possessed of little organization 

 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 

 inaple and thorn it is principally con- 

 stituted 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 full of 

 cavities, and it is probably altogether by 

 this surface that moisture and the princi- 

 ples 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 posi- 

 tion; and all leaves elevate themselves on 

 the foot stalk during their exposure to the 

 solar light, and, as it were, move toward 

 the sun. 



This effect seems in a great measure 

 dependant upon the mechanical and 

 chemical agency 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 surface, 

 turned exactly in the same manner as the 

 natural leaves. 



This, however, can be considered only 

 as a very rude imitation of the natural 

 process. 



What Linnaeus has called the sleep of 

 the leaves, appears to depend wholly 

 upon the defect of the action of light and 

 heat, and the excess of the operation of 

 moisture. 



This singular but constant phenomenon 

 had never been scientifically observed, till 

 the attention of the botanist of Upsal was 

 fortunately directed to it. He was ex- 

 amining particularly a species of lotus, in 

 which four flowers had appeared during 

 the day, and he missed two in the even- 

 ing ; by accurate inspection he soon dis- 

 covered 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 the garden and witnessed a series of 

 curious facts before unknown. All the 

 simple leaves of the plants he examined, 

 had an arrangement totally different from 

 their arrangement 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. 

 DecandoUe made this experiment on the 

 sensitive plant. By confining it in a dark 

 place in the day time, the leaves soon 

 closed; but on illuminating the chamber 

 with 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 annually decay, and are reproduced; 

 their decay takes place either at the con- 

 clusion of summer, as in very hot climates, 

 when they are no longer supplied with 

 sap, in consequence of the dryness of soil 

 and the evaporating powers of heat; or 

 in the autumn, as in the northern climates 

 at the commencement 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 de- 

 pend upon the nature of the chemical 

 change, and the acids are generally de- 

 veloped, 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 

 leaves 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 the plants of this species, and 

 probably there is a certain degree of 

 circulation throughout the winter; their 

 juices are less watery than those of other 

 plants, and probably less liable to be con- 

 gealed by cold, and they 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 vigorously performing their 

 functions. If the leaves are stripped off 



