256 



TISSUES 



but with thinner walls and the central cavity not so 

 nearly closed. In some cases such fibers have transverse 

 walls. Wood cells constitute a large part of the wood of 

 some plants and are in other cases found scattered only 

 among the other prosenchyma. (3) Tracheids. Cells of 

 this tissue differ from ordinary cells in being supplied with 

 numerous bordered pits or other characteristic markings. 



398. Longitudinal tangentiiil section of Scotch pine wood, liighly magnified. 

 It shows tracheids with bordered pits. The dark cells are ends of medullary rays. 



They constitute the largest part of the wood of the pines 

 and other gymnosperms. Fig. 398. (4) Vascular tissue, 

 composed of large cells which become confluent end to end, 

 forming long tubes or ducts. TT', Fig. 401. From the 

 thickened markings which these cells bear they are named 

 spiral, annular, pitted, scalariform, etc. Fig. 389. These 

 vessels are often of considerable length, but are never con- 

 tinuous through the entire plant. Cut a grape-vine stem 

 2 or 3 feet long. Place one cut end in a glass of water and 

 with the other end in the mouth, try to force air through 

 the stem. If not successful, shorten the stem a little. 



