80 



VIEWS OF THE MICROSCOPIC WORLD. 



Fig. 125 



around the pith in concentric layers, and rays of cellular tissue to run from the 

 pith to the bark, diverging like the spokes of a wheel from the axle. This arrange- 

 ment is seen in drawing 134, which represents a cross section of part of an 

 ash branch three years old. The numerous vessels of the wood, vhieh are de- 

 noted by small circles, are here seen occupying the space D C I K ; from the 

 pith I K L to the bark A B C D ; and the insertions of the cellular tissue are indi- 

 cated by the lines that run from the pith outward like the sticks of a fan. These 

 insertions of tissue pass through the substance of the wood, and are much diver- 

 sified in size in different woods. In pine they are of a medium size, and in pear 

 and holly extremely small, but no uniformity in this respect is observed in the 

 same wood, for in holly, hazel, pear, plum, and oak, they 

 are very unequal ; some in the holly being four or five 

 times thicker than the rest; while in the plum many are 

 six or seven times greater than others, and in the oak ten 

 times at the least. In trees like the palm, the vessels of the 

 vascular system are by no means so numerous as in other 

 woods, and being necessarily placed at a greater distance 

 from each other, do not present that symmetrical radi- 

 ated appearance which sections of common trees exhibit; 

 but the bundles of tubes are promiscuously scattered amid 

 the cellular tissue. This is evident from a glance at figure 

 125, which represents a cross section of the palm, the dark 

 spots indicating the position of the vessels, and the lighter 

 the cellular tissue. In the case of herbs, to a great extent, 

 the cellular tissue forms the chief portion of the plant, and 

 the vessels of the vascular system are but few in number. 

 When a cross section is viewed they are seen in bundles dis- 

 persed through the cellular tissue, at considerable distances 

 from each other ; they are, however, symmetrically ar 

 ranged, and in the same species of plant always maintain 

 the same position ; the vessels being situated at the same relative distance from 

 each other and from the centre of the pith. 



BARK. This envelope, which encircles the wood, is composed of two parts, the 

 true bark and the outer skin which covers it; both of which are made up of ves- 

 sels and cellular tissue like the wood. The tubes or vessels belonging to the 

 bark are denominated proper vessels, and are filled with the fluids peculiar to this 

 portion of the plant. In some herbs these vessels often cluster together in separate 

 columns, and are arranged in a circular form ; in others they present a radiated 

 appearance. In trees, the tubes are more distinct, and assume a greater regu- 

 larity in their disposition. 



They are usually found near the inner margin of the bark, next to the wood, 

 and when viewed in the direction of their length, present an appearance like net- 

 work. In the bark the vessels are found to pos&ess different sizes as well as in 



