STRUCTURE OF VEGETABLES. 113 



and the elephant do not differ from each other more in size, 

 v nan they do in the number and complexity of their organs, 

 and the extent and perfection of their powers and functions. 

 Whilst, on the other hand, the humblest moss is scarcely less 

 perfect, less complicated, or possessed of less extensive pow- 

 ers, than the most lofty tree. Vegetables vary in their form, 

 .heir size, their fruit, and many other particulars ; but we 

 cannot say of one, that it is of a higher order in the scale of 

 creation than another, as we can of animals. There is no 

 series beginning at an individual of low and obscure powers, 

 and ending in one of powers numerous and elevated. 



The structure of plants is exceedingly simple, and nearly 

 alike in all the different classes, so far as they have been ex- 

 amined. The important parts, which serve to provide for 

 their nourishment and growth, are the root, the stem, and 

 the leaves. These are all formed of a variety of vessels and 

 tubes, in which the sap and other fluids circulate. The sap 

 is in the first place taken into the roots, probably by means 

 of the long fibrous filaments which are usually extended in 

 every direction, and conveyed into what are called the cen- 

 tral vessels. These are so called from their being arranged, 

 in annual shoots and herbaceous plants, around the pith or 

 centre of the stem. They are constructed of fibres which 

 are wound spirally around them, and are thence sometimes 

 called the spiral vessels. This arrangement probably con- 

 tributes to the motion of the fluids they contain. By these 

 vessels the sap is carried up the stem and distributed to-' the 

 different branches, and thence to the leaves, flowers, or fruit. 

 In the leaves it circulates, and is there exposed to the influ- 

 ence of light, heat, and air; and is no doubt perfected and 

 elaborated by the processes which it undergoes. Returning 

 from the leaves, it descends in a different set of vessels situ- 

 ated in the bark, and in its descent contributes to the growth 

 and nourishment of the plant by depositing new layers of vege- 

 table matter between the bark and the wood. 



This is the course o. the circulation in annual plants, in 

 which there is an entirely new growth every year from the 

 root. But in trees and shrubs where the same trunk or stem 

 continues from year to year, the arrangement, though essen- 

 tially the same, is a little varied. The central vessels are 

 not situated directly around the pith, but in the external layt ' 

 of wood called the alburnum, which is always the growth 

 of the preceding year, and performs this office only for one 

 season, being afterwards surrounded and inclosed by a new 



