SIEVE-TUBES 345 



might be strengthened by gumming a spiral strip of paste- 

 board to its inner surface. These vessels are of considerable 

 length, and are open at both ends : moreover they contain 

 no protoplasm, but are filled with either air or water : they 

 have therefore none of the characteristics of cells. They 

 are show r n, by treatment with Schulze's solution, to be com- 

 posed of lignin. 



Surrounding the group of spiral vessels, and forming the 

 large polygonal meshes so obvious in a transverse section, 

 are wide tubes (B and c, sc. v) pointed at both ends and 

 fitting against one another in longitudinal series by their 

 oblique extremities. They have transverse markings like 

 the rungs of a ladder, and are hence called scalariform 

 vessels. The markings (i) are due to wide transverse pits 

 in the otherwise thick lignified walls : in the oblique ends 

 by which the vessels fit against one another, the pits are 

 frequently replaced by actual slits, so that a longitudinal 

 series of such vessels forms a continuous tube containing, 

 like the spiral vessels, air or water, but no protoplasm. In 

 most ferns the terminal walls are not thus perforated, and 

 the elements are then called tracheides. 



The presence of these vessels spiral and scalariform- 

 is the most important histological character separating ferns 

 and mosses. The latter group and all plants below them are 

 composed exclusively of cells : ferns and all plants above 

 them contain vessels in addition, and are hence called vas- 

 cular plants. 



The vessels, together with small parenchyma-cells inter- 

 spersed among them, make up the central portion of the 

 vascular bundle, called the wood or xylem. The peripheral 

 portion is formed of several layers of cells composing the bast 

 or phloem, and surrounding the whole is a single layer of 

 small cells, the bundle-sheath (p. s/i). 



