SIR H. DAVY S AGRICULTURAL CHEMISTRY. 



105 



The silver grain is elastic and con- 

 tractile, and it has been supposed by Mr. 

 Knight, that the change of volume pro- 

 duced in it by change of temperature is 

 one of the principal causes of the ascent 

 of the sap. The fibres of it seem always 

 to expand in the morning and contract 

 at night ; and the ascent of the juices, as 

 was stated in the last lecture, depends 

 principally on the agency of heat. The 

 silver grain is most distinct in forest trees; 

 but even annual shrubs have a system of 

 fibres similar to it. The analogy of 

 nature is constant and uniform, and 

 similar effects are usually produced by 

 similar organs. 



The pith occupies the centre of the 

 wood, its texture is membranous ; it is 

 composed of cells, which are circular 

 towards the extremity', and hexagonal in 

 the centre of the substance. In the first 

 infancy of the vegetable, the pith occupies 

 but a small space. It gradually dilates, 

 and in annual shoots and young trees 

 offers a considerable diameter. In the 

 more advanced age of the tree, acted on 

 by the heart-wood, pressed by the new 

 layers of the alburnum, it begins to 

 diminish, and in very old forest trees 

 disappears altogether. 



Many different opinions have prevailed 

 with regard to the use of pith. Dr. Hales 

 supposed, that it was the great cause of 

 the expansion and development of the 

 other parts of the plants, that being the 

 most interior, it was likewise most acted 

 upon of all the organs, and that from its 

 reaction the phenomena of their growth 

 resulted. 



Linnaeus, whose lively imagination was 

 continually employed in endeavors to 

 discover analogies between the animal 

 and vegetable systems, conceived " that 

 the pith performed for the plant the same 

 functions as the brain and nerves in an- 

 imated beings." He considered it as the 

 organ of irritability and the seat of life. 



The latest discoveries have proved, that 

 these two opinions are equally erroneous. 

 Mr. Knight has removed the pith in 

 several young trees and they continued 

 to live and to increase. 



It is evidently then only an organ of 

 secondary importance. In early shoots, 

 in vigorous growth it is filled with mois- 



ture, and it is a reservoir, perhaps, of 

 fluid nourishment, at the time it is most 

 wanted. As the heart-wood forms, it is 

 more and more separated from the living 

 part, the alburnum; its functions become 

 extinct, it diminishes, dies, and at last dis- 

 appears. 



The tendrils, the spines, and other 

 similar parts of plants are analogous in 

 their organization to the branches, and 

 offer a similar cortical and alburnous or- 

 ganization. It has been shown, by the 

 late observations of Mr. Knight, that the 

 directions of tendrils, and the spinal form 

 they assume depend upon the unequal 

 action of light upon them, and a similar 

 reason has been assigned by Mr. Decan- 

 dolle to account for the turning of plants 

 towards the sun ; that ingenious physio- 

 logist supposes that the fibres are short- 

 ened 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 perform the same functions. 



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 peculiar tubes, 

 particularly the tracheas, may be distinctly 

 seen in the leaf. 



The green membranous substance may 

 be considered as an extension of the 

 parenchyma, and the fine and thin cover- 

 ing as the epidermis. Thus the organiza- 

 tion of the roots and branches may be 

 traced into the leaves, which present, 

 however, a more perfect, refined and 

 minute structure. 



The great use of the leaves is for the 

 exposure of the sap to the influence of ihe 

 air, heat, and light. Their surface is ex- 

 tensive, 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 fitted for its organizing 

 functions, and probably passes in its pre- 

 pared state, from the extreme tubes of the 

 alburnum into the ramifications of the 

 cortical tubes, and then descends through 

 the bark. 



