SYSTEMS OF TISSUES AND PLANT GROWTH. 



163 



413. This book is intended to consider the higher 

 plants only ; we shall therefore now proceed to describe 

 the manner in which the modi- 

 fied cells and vessels are ar- 

 ranged in the higher organized 



plant structures. 



414. Exogenous or Dicotyle- 

 donous structure. Growth in the 

 most highly organized plants is 

 best illustrated by the examina- 

 tion of a tree or shoot of Oak, 



Maple, Apple, or Cherry at the end of the first year of 

 its life. A cross-section of such a 

 scion presents a circle of pith in 

 the center, around which are con- 

 centric circular rings, the inner one 

 wood, the outer ones bark. In the 

 figure (514), a, the pith; 5, the 

 wood ; c, the bark. On the inner 

 edge of the wood is a ring of spi- 

 ral vessels, d, which is called the 

 medullary or pith sheath. The pith 

 is made up of parenchyma and 

 extends between the wedges of 

 wood in flat cells connecting the 

 pith with the bark (1, 2, 3, Fig. 

 515, A, cross-section of the stem of 514), forming the silver grain 



a Maple at the end of the first season's . /--\i i /r i 11 



growth; i, edge of the pith; 2, spiral seen in Oak and Maple planks, 



vessels: 3, wood region made up of . , i i i j.' t 



woody fiber and dotted ducts and other Or III a longitudinal SCCtlOn Ol 

 vessels; 4, camhium layer made up of , ,, -, . j 



new cells; 5, liber: 6, larger cells and thOS6 and Other Cabinet WOOQS 

 vessels of the liber or bast region; 7, 



cellular envelope or green bark; 8, When Split. 



corky envelope or outer bark ; 9, the ., 



skin, or epidermis. B, shows corre- 415. The WOOQ IS made Up OI 



sponding vessels and tissues in a ver- 

 tical section of the same plant, woody fiber interspersed with tis- 



