258 VEGETABLE. 



fluids into others of the most opposite qualities, the delicate 

 membranes of which they are composed, constituting a suf- 

 ficient barrier to prevent their mingling with each other. 

 But as soon as life is extinct, the sides of the cells yield , 

 the secretions mingle, and decomposition succeeds, during 

 which, the chemical affinities, which had been controlled by 

 vital action, produce disorganization, and decay. 



Spiral Vessels of Plarits. We have seen, that in the lower 

 orders of vegetables, the interior structure is composed 

 chiefly or entirely of cells, or small compartments, separated 

 by membranes. In phaenogamous plants, although cells are 

 still found, the chief organic structure consists of continuous 

 vessels made up of threads, wound in the form of a screw, 

 and hence they are called spiral organs. 



Conducting and Returning vessels. In describing the 

 structure of plants, it will be convenient to arrange their 

 vessels, according to their functions, or as Conducting and 

 Returning organs. 



If the branch of a vine, or any other tree, be cut trans- 

 versely in the early spring, the sap may be observed to trans- 

 ude, from numerous points, over the whole cut surface, except 

 at those parts occupied by the pith and bark ; and if a twig 

 on which the leaves are just beginning to unfold, be cut from 

 a tree, and placed with its lower end in a watery solution of 

 some coloring matter, as that of the Brazil wood, the colored 

 fluid will ascend to the top of the twig, and into the leaves. 

 In both these cases, an examination with a powerful micro- 

 scope will demonstrate that the sap, and colored water, rise 

 through organic tubes, and which must therefore be in conse- 

 quence of the action of the living vegetable. These are 

 the conducting vessels of the plant, which carry the sap 

 upwards. 



But if we examine the transverse section of another twig 

 of the same plant, at a later period of the season, we shall 

 find that the wood, which before became moist on the cut 

 surface, remains dry there, whilst the bark next to the wood 

 is full of fluid, which instantly begins to exude. The vessels 

 from which the sap now comes, are a different series from 

 those by which it was conducted upwards, and are termed 

 returning vessels, from their function of carrying the sap 

 downwards, after its elaboration in the leaves. 



The general figure of both the Conducting and the Re- 

 turning vessels is cylindrical. They are so minute as to 

 be hardly visible to the naked eye. In a piece of oak, of 



