20 FUNCTIONS OF VESSELS. 



plete spiral fibre in one part, annular fibres, either complete or with their 

 ends overlapping, at another, and bars or dots at a third portion. In the 

 case of some vessels, their formation can be distinctly traced to cells 

 placed end to end, the partitions between which have been absorbed. 

 The moniliform or beaded appearance often presented by the different 

 kinds of vessels, more especially the Porous, plainly indicates this mode 

 of formation. 



40. As in cells, so in vessels, the walls are composed of cellulose, and 

 there are usually no visible perforations; the communication between 

 them taking place by imbibition or endosmose. In some instances, 

 when vessels are closely applied to each other, especially when they 

 overlap, the membrane becomes absorbed, and direct communication 

 takes place. This has been seen in spiral and porous vessels. The pits 

 or depressions on the walls of vessels, and the thinning of the tissue 

 at particular points, appear to serve the purpose of allowing the rapid 

 transmission of fluids ; and, according to some, they permit the passage 

 of small cells from the interior, which become developed as tubes, and 

 form branching vessels. 



41. Pleurenchyma, in its early state, contains fluids, and conveys 

 them from one part of the plant to another. In the progress of growth, 

 the secondary deposits obliterate the vessels, as in the perfect or 

 heart wood of ordinary trees. These deposits are often of a very 

 hard nature, and assume particular colours in different kinds of trees. 

 From the firmness of this tissue, it is well fitted to give solidity to 

 the stems and to strengthen the leaves of plants. In Spiral vessels, 

 the fibre adds to their elasticity, and keeps the tubes always pervious. 

 The fibre, when once formed, does not increase much in thickness, and 

 the secondary deposits do not obliterate the canal. Various opinions 

 have prevailed regarding the contents of these vessels. The name 

 Trachea;, given by Grew and others, was partly from their structure, 

 and partly from the idea that they contained air. The accurate experi- 

 ments of Bischoff lead to the conclusion that the perfect spiral vessels 

 convey air, which often contains a large amount of oxygen in its com- 

 position. Hales showed that air was evolved from the vessels of the 

 Vine when cut, and Decandolle thought that part of the air in these 

 vessels was derived from the pores of the leaves. Other authors look 

 upon these vessels as conveying fluids/while a third set maintains that 

 both air and fluids are present, the air being derived in part from 

 decompositions going on in the interior of the plant. The other kinds 

 of vascular tissue, and especially the porous vessels, are the means 

 by which the fluids taken up by the roots of plants are conveyed to 

 the leaves, and to all parts of the plants. Laticiferous vessels contain, 

 according to Schultz, the elaborated sap or latex on its return from 

 the leaves to the bark. This latex is either transparent or opaque, 

 colourless or coloured. These vessels, when examined with the micro- 



