WOODY AND VASOULAB TISSUE. 31 



disk. These circular markings, or disks are found in single, 

 double, and triple rows. When there is more than one row, 

 the individual disks are either opposite to each other, as in 

 ordinary pine wood, or they alternate with each other, as in 

 Araucarias. Mr. William Nichol, of England, and Prof. 

 Bailey, of West Point, were the first to apply these character.? 

 to the determination of fossil woods, thus revealing the true 

 nature of some of the earliest vegetation of the earth.* 



30. Plants are durable and strong in exact proportion to 

 the amount of woody fibre which enters into their composition. 

 The lowly mosses and other orders of cellular plants scarcely 

 rise more than a few inches above the ground j but when 

 w;oody fibre begins to form amongst the cells, additional 

 strength is given to the vegetable fabric, and plants are de- 

 veloped into herbs, shrubs, and assume the noblest arbores- 

 cent forms. 



31. Vascular tissue. — This consists of a series of vesicles of 

 cellular tissue, having fibre generated spirally in their inside, 

 drawn out into membranous tubes tapering to either extremity, 

 the walls of the tubes being absorbed at the points where they 

 overlap each other, so as to allow of direct communication be- 

 tween them. More frequently, however, these tubes, instead 

 of tapering, terminate abruptly at their extremities, and when 

 this is the case, their origin from a series of elongated cells, 

 placed end to end, is often beautifully apparent in the remains 

 of the persistent parietes of the cells which are seen along the 

 cavity of the tube, and by which its continuity is not unfre- 

 quently interrupted. 



* Balfour's Class-Book of Botany. 



