22 FUNCTIONS OF VESSELS. 



produced in vascular plants. To the elongation of cells, and the 

 deposition of thickening layers and fibres in their interior, the various 

 vessels owe their origin. Thus when cells are elongated, as spindle- 

 shaped tubes, and their walls are thickened and hardened by depo- 

 sitions of ligneous matter, they give rise to Pleurenchyma ; and when 

 elongated membranous tubes are strengthened by spiral fibres, the 

 difierent kinds of Fibro-vascular tissue are produced. The spiral 

 vessel may be considered as the type of the last-mentioned tissue, 

 and aU its varieties may be traced to difierent conditions in de- 

 velopment of the fibre. In the case of some vessels, their forma- 

 tion can be distinctly traced to cells placed end to 

 end, the partitions between which have been ab- 

 sorbed. The raoniliform or beaded appearance often 

 presented by the different kinds of vessels, more espe- 

 cially the Pitted, plainly indicates this mode of for- 

 mation. Occasionally cellular prolongations are seen 

 in the interior of pitted vessels, giving rise to what 

 has been called Tylosis (ruXos, swelling or protru- 

 sion). It has been noticed in the vessels of Oak, 

 Kg- n. Chestnut, Wahiut (fig. 71 a), Ash, Elm, etc. 



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 osmose. 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 pitted 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. 



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. 

 Prom the flrmness 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 

 Tracheae, given by Grew and others, was partly from their structure, 

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

 experiments of BischofiF lead to the conclusion that the perfect spiral 



Fig. 71. Longitudinal section of the stem of a species of Walnut (Juglans einerea), showing 

 ylosis in pitted vessels, a. 



