164 NATUEA.L SYSTEM. 



and yapos, concealed marriage, because no parts of fructifica- 

 tion have yet been discovered The other division comprises 

 such as are more complicated in structure, being furnished 

 with cellular tissue and tubular vessels, and the embryo having 

 one or more cotyledons, whence they are called vascular or 

 cotyledonous, and are subdivided into the dicotyledonous, or 

 exogenous, and the monocotyledonous, or endogenous classes. 



EXOGENOUS OK DICOTYLEDONOUS STRUCTURE. A trans- 

 verse section of a branch of the oak (fig. 99) exhibits a 



FIG. 99. Transverse section of dicotyledonous wood. 



central, cellular substance or pith ; an external cellular and 

 fibrous ring, or bark ; an intermediate woody mass, and certain 

 fine lines, radiating like the spokes of a wheel, from the pith 

 to the bark through the wood, and called medullary rays. 



FIG. 100 Germ of a dicotyledonous plant. 



This is the structure of an exogenous plant, so called from 

 f|<, without, and ytwav, to increase, because its mode of 

 growth is by fresh layers from without, from the circum- 

 ference to the centre ; it is also termed dicotyledonous, 

 because its germ has two cotyledons, or seed-leaves (fig. 100). 



