184 Elementary Biology. 



number of cells enclosing a cavity, which is thus an inter- 

 cellular space. They also contain substances of economic 

 value, as turpentine, resin, &c. 



Numerous other varieties of vessels, ducts, and glands 

 might be mentioned, such as oil glands, nectar glands, 

 cavities containing volatile oils, and such like. The pecu- 

 liar glands of insectivorous plants will be described in 

 sect. vi. 



FIG. 98. TRANSVERSE SECTION OF A RESIN CANAL. (Vines.) 



Root. The chief point of importance in the dicotyledon- 

 ous root in which it differs from that of the monocotyledon 

 is its power of increasing in thickness by growth of second- 

 ary wood. It will be remembered that while the primary 

 root of the monocotyledon soon ceases to grow, and has its 

 place taken by adventitious roots, the primary root of the 

 dicotyledon goes on growing, and increases in thickness by 



